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Condended Financial Information of Registrant Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
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
Condensed Statement of Income Captions [Line Items]                      
Net income (loss) $ 19,005 $ 18,824 $ 7,006 $ 27,786 $ 13,687 $ (27,761) $ (13,088) $ (18,673) $ 72,621 $ (45,835) $ (39,911)
Other comprehensive income (loss):                      
Reclassification of other post-retirement benefits loss to net income                 0 0 1,082
Other post-retirement benefits income (loss)                 (52) 2,196 (636)
Total other comprehensive income (loss), net of tax                 (52) 2,196 446
Comprehensive income (loss)                 72,569 (43,639) (39,465)
Parent Company                      
Condensed Statement of Income Captions [Line Items]                      
Net income (loss)                 72,621 (45,835) (39,911)
Other comprehensive income (loss):                      
Reclassification of other post-retirement benefits loss to net income                 0 0 1,082
Other post-retirement benefits income (loss)                 (52) 2,196 (636)
Total other comprehensive income (loss), net of tax                 (52) 2,196 446
Comprehensive income (loss)                 $ 72,569 $ (43,639) $ (39,465)