Consolidated Statements of Stockholder's Equity - USD ($) $ in Thousands	Total	Common  Stock	Additional  Paid-in Capital	Retained Earnings	Retained Earnings ASU 2016-01	Retained Earnings ASU 2018-02	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment		Accumulated Other   Comprehensive  Income	Accumulated Other   Comprehensive  Income ASU 2016-01	Accumulated Other   Comprehensive  Income ASU 2018-02	Total Equity	Total Equity ASU 2016-01	Total Equity ASU 2018-02	Total Equity Cumulative Effect, Period of Adoption, Adjustment
Beginning Balance at Dec. 31, 2017		$ 2,500	$ 1,141,092	$ 1,526,310	$ 7,936	$ (30,449)			$ 165,346	$ (1,539)	$ 30,449	$ 2,835,248	$ 6,397	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201601Member
Contributed capital			5,500									5,500
Dividend to parent				0								0
Contributed (distributed) capital-parent/child asset transfers			0									0
Comprehensive income:
Net income (loss)	$ 108,638			108,638								108,638
Other comprehensive income (loss), net of tax	$ (222,552)								(222,552)			(222,552)
Total comprehensive income (loss)												(113,914)
Ending Balance at Dec. 31, 2018		2,500	1,146,592	1,612,435			$ (1,981)	[1]	(28,296)			2,733,231			$ (1,981)	[1]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201708Member
Contributed capital			5,900									5,900
Dividend to parent				(250,000)								(250,000)
Contributed (distributed) capital-parent/child asset transfers			1,140									1,140
Comprehensive income:
Net income (loss)	$ 216,999			216,999								216,999
Other comprehensive income (loss), net of tax	$ 309,738								309,738			309,738
Total comprehensive income (loss)												526,737
Ending Balance at Dec. 31, 2019		2,500	1,153,632	1,577,453			$ (1,752)	[2]	281,442			3,015,027			$ (1,752)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Contributed capital			575,000									575,000
Dividend to parent				0								0
Contributed (distributed) capital-parent/child asset transfers			(1,942)									(1,942)
Comprehensive income:
Net income (loss)	$ 196,697			196,697								196,697
Other comprehensive income (loss), net of tax	$ 264,686								264,686			264,686
Total comprehensive income (loss)												461,383
Ending Balance at Dec. 31, 2020		$ 2,500	$ 1,726,690	$ 1,772,398					$ 546,128			$ 4,047,716

[1] Includes the impact from the adoption of ASU 2017-08 and 2017-12.

[2] Includes the impact from the adoption of ASU 2016-13. See Note 2.