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	$ 1,437,266	$ 70,975	$ 2,496,803
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			5,000			5,000
Comprehensive income (loss):
Net income (loss)	$ 40,072			40,072		40,072
Other comprehensive income (loss), net of tax	17,697				17,697	17,697
Total comprehensive income (loss)						57,769
Ending Balance at Mar. 31, 2017		2,500	991,062	1,477,338	88,672	2,559,572
Beginning Balance at Dec. 31, 2017		2,500	1,141,092	1,511,698	165,346	2,820,636
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				(30,399)	30,399	0
Contributed capital			5,500			5,500
Comprehensive income (loss):
Net income (loss)	10,116			10,116		10,116
Other comprehensive income (loss), net of tax	$ (115,274)				(115,274)	(115,274)
Total comprehensive income (loss)						(105,158)
Ending Balance at Mar. 31, 2018		$ 2,500	$ 1,146,592	$ 1,499,351	$ 78,932	$ 2,727,375