Unaudited Interim Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total Equity
Beginning Balance at Dec. 31, 2016		$ 2,500	$ 986,062	$ 1,448,517	$ 70,975	$ 2,508,054
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			5,000			5,000
Contributed (distributed) capital-parent/child asset transfers			875			875
Comprehensive income (loss):
Net income (loss)	$ 387,069			387,069		387,069
Other comprehensive income (loss), net of tax	84,153				84,153	84,153
Total comprehensive income (loss)						471,222
Ending Balance at Jun. 30, 2017		2,500	991,937	1,835,586	155,128	2,985,151
Beginning Balance at Dec. 31, 2017		2,500	1,141,092	1,526,310	165,346	2,835,248
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			5,387			5,387
Comprehensive income (loss):
Net income (loss)	47,530			47,530		47,530
Other comprehensive income (loss), net of tax	$ (184,581)				(184,581)	(184,581)
Total comprehensive income (loss)						(137,051)
Ending Balance at Jun. 30, 2018		$ 2,500	$ 1,146,479	1,551,327	9,675	2,709,981
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption | ASU 2016-01				7,936	(1,539)	6,397
Cumulative effect of adoption | ASU 2018-02				$ (30,449)	$ 30,449	$ 0