Annual report pursuant to Section 13 and 15(d)

New Accounting Standards (Details)

v3.8.0.1
New Accounting Standards (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jan. 01, 2018
Dec. 11, 2017
Oct. 03, 2017
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 04, 2016
Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle                                
Operating income       $ 27,882,000 $ 46,558,000 $ 55,149,000 $ 37,915,000 $ 15,139,000 $ 44,254,000 $ 16,749,000 $ 8,048,000 $ 167,504,000 $ 84,190,000 $ 55,775,000    
Non operating income                       (25,980,000) 3,402,000 $ 0    
Restricted cash       $ 1,048,000       $ 14,335,000       1,048,000 14,335,000      
Unsecured Debt | Floating Rate Senior Unsecured Notes due 2022                                
New Accounting Pronouncements or Change in Accounting Principle                                
Debt extinguishment costs   $ 6,043,000 $ 32,284,000                 $ 47,875,000 $ 0      
Debt instrument face amount     525,000,000                       $ 525,000,000  
Debt extinguishment amount   78,792,000 $ 446,208,000                          
Senior Notes | 8.5% Senior Notes due 2022                                
New Accounting Pronouncements or Change in Accounting Principle                                
Debt extinguishment costs   $ 7,996,000                            
Debt instrument face amount                               $ 200,000,000
Debt instrument stated interest rate       8.50%               8.50%       8.50%
Pro Forma | ASU 2017-07                                
New Accounting Pronouncements or Change in Accounting Principle                                
Operating income $ (2,651,000)                              
Non operating income $ 2,651,000