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			153,500			153,500
Contributed (distributed) capital-parent/child asset transfers			875			875
Comprehensive income (loss):
Net income (loss)	$ 209,873			209,873		209,873
Other comprehensive income (loss), net of tax	77,698				77,698	77,698
Total comprehensive income (loss)						287,571
Ending Balance at Sep. 30, 2017		2,500	1,140,437	1,658,390	148,673	2,950,000
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			5,500			5,500
Comprehensive income (loss):
Net income (loss)	66,255			66,255		66,255
Other comprehensive income (loss), net of tax	$ (215,006)				(215,006)	(215,006)
Total comprehensive income (loss)						(148,751)
Ending Balance at Sep. 30, 2018		$ 2,500	$ 1,146,592	1,570,052	(20,750)	2,698,394
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