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
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)	$ 66,255
Other comprehensive income (loss), net of tax	(215,006)
Ending Balance at Sep. 30, 2018		2,500	1,146,592	1,570,052	(20,750)	2,698,394
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)
Comprehensive income (loss):
Net income (loss)				36,226		36,226
Other comprehensive income (loss), net of tax					(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]
Contributed capital			113			113
Comprehensive income (loss):
Net income (loss)	18,725			18,725		18,725
Other comprehensive income (loss), net of tax	(30,425)				(30,425)	(30,425)
Total comprehensive income (loss)						(11,700)
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					(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 (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)	179,596
Other comprehensive income (loss), net of tax	357,390
Ending Balance at Sep. 30, 2019		2,500	1,147,732	1,790,917	329,094	3,270,243
Beginning Balance at Mar. 31, 2019		2,500	1,146,588	1,699,109	104,205	2,952,402
Comprehensive income (loss):
Net income (loss)				48,435		48,435
Other comprehensive income (loss), net of tax					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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed (distributed) capital-parent/child asset transfers			1,144			1,144
Comprehensive income (loss):
Net income (loss)	43,373			43,373		43,373
Other comprehensive income (loss), net of tax	$ 78,929				78,929	78,929
Total comprehensive income (loss)						122,302
Ending Balance at Sep. 30, 2019		$ 2,500	$ 1,147,732	$ 1,790,917	$ 329,094	$ 3,270,243