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			0
Comprehensive income (loss):
Net income (loss)				11,304		11,304
Other comprehensive income (loss), net of tax					(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
Beginning Balance at Dec. 31, 2017		2,500	1,141,092	1,526,310	165,346	2,835,248
Comprehensive income (loss):
Net income (loss)	$ 47,530
Other comprehensive income (loss), net of tax	(184,581)
Ending Balance at Jun. 30, 2018		2,500	1,146,479	1,551,327	9,675	2,709,981
Beginning 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]
Contributed capital			(113)			(113)
Contributed (distributed) capital-parent/child asset transfers			0			0
Comprehensive income (loss):
Net income (loss)	36,226			36,226		36,226
Other comprehensive income (loss), net of tax	(69,307)				(69,307)	(69,307)
Total comprehensive income (loss)						(33,081)
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					(1,539)
Cumulative effect of adoption | ASU 2018-02					30,449
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
Other comprehensive income (loss), net of tax					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
Beginning Balance at Dec. 31, 2018		2,500	1,146,592	1,612,435	(28,296)	2,733,231
Comprehensive income (loss):
Net income (loss)	136,223
Other comprehensive income (loss), net of tax	278,461
Ending Balance at Jun. 30, 2019		2,500	1,146,588	1,747,544	250,165	3,146,797
Beginning 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]
Contributed capital			0			0
Contributed (distributed) capital-parent/child asset transfers			0			0
Comprehensive income (loss):
Net income (loss)	48,435			48,435		48,435
Other comprehensive income (loss), net of tax	$ 145,960				145,960	145,960
Total comprehensive income (loss)						194,395
Ending Balance at Jun. 30, 2019		$ 2,500	$ 1,146,588	$ 1,747,544	$ 250,165	$ 3,146,797