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, 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
Contributed (distributed) capital-parent/child asset transfers				0
Comprehensive income (loss):
Net income (loss)		$ 11,304			11,304		11,304
Other comprehensive income (loss), net of tax		(115,274)				(115,274)	(115,274)
Total comprehensive income (loss)							(103,970)
Ending Balance at Mar. 31, 2018			2,500	1,146,592	1,515,151	78,932	2,743,175
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
Cumulative effect of adoption		(27)
Beginning Balance at Dec. 31, 2018			2,500	1,146,592	1,612,435	(28,296)	2,733,231
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital				0			0
Contributed (distributed) capital-parent/child asset transfers				(4)			(4)
Comprehensive income (loss):
Net income (loss)		87,788			87,788		87,788
Other comprehensive income (loss), net of tax		$ 132,501				132,501	132,501
Total comprehensive income (loss)							220,289
Ending Balance at Mar. 31, 2019			$ 2,500	$ 1,146,588	1,699,109	104,205	2,952,402
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption	[1]				$ (1,114)	$ 0	$ (1,114)

[1] (1)Includes the impact from the adoption of ASUs 2017-08 and 2017-12. See Note 2.