Unaudited Interim Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common   Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive   Income (Loss)	Cumulative effect of adoption of accounting changes		Cumulative effect of adoption of accounting changes Retained Earnings
Beginning Balance at Dec. 31, 2018	$ 2,733,231	$ 2,500	$ 1,146,592	$ 1,612,435	$ (28,296)	$ (1,114)	[1]	$ (1,114)	[1]
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	2,952,402	2,500	1,146,588	1,699,109	104,205
Beginning Balance at Dec. 31, 2018	2,733,231	2,500	1,146,592	1,612,435	(28,296)	(1,114)	[1]	(1,114)	[1]
Comprehensive income (loss):
Net income (loss)	179,596
Other comprehensive income (loss), net of tax	357,390
Ending Balance at Sep. 30, 2019	3,270,243	2,500	1,147,732	1,790,917	329,094
Beginning Balance at Mar. 31, 2019	2,952,402	2,500	1,146,588	1,699,109	104,205
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
Other comprehensive income (loss), net of tax	145,960				145,960
Total comprehensive income (loss)	194,395
Ending Balance at Jun. 30, 2019	3,146,797	2,500	1,146,588	1,747,544	250,165
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital	0		0
Contributed (distributed) capital-parent/child asset transfers	1,144		1,144
Comprehensive income (loss):
Net income (loss)	43,373			43,373
Other comprehensive income (loss), net of tax	78,929				78,929
Total comprehensive income (loss)	122,302
Ending Balance at Sep. 30, 2019	3,270,243	2,500	1,147,732	1,790,917	329,094
Beginning Balance at Dec. 31, 2019	3,015,027	2,500	1,153,632	1,577,453	281,442	(1,752)	[2]	(1,752)	[2]
Comprehensive income (loss):
Net income (loss)	97,612			97,612
Other comprehensive income (loss), net of tax	(126,381)				(126,381)
Total comprehensive income (loss)	(28,769)
Ending Balance at Mar. 31, 2020	2,984,506	2,500	1,153,632	1,673,313	155,061
Beginning Balance at Dec. 31, 2019	3,015,027	2,500	1,153,632	1,577,453	281,442	$ (1,752)	[2]	$ (1,752)	[2]
Comprehensive income (loss):
Net income (loss)	257,222
Other comprehensive income (loss), net of tax	173,641
Ending Balance at Sep. 30, 2020	3,842,196	2,500	1,551,690	1,832,923	455,083
Beginning Balance at Mar. 31, 2020	2,984,506	2,500	1,153,632	1,673,313	155,061
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital	325,000		325,000
Contributed (distributed) capital-parent/child asset transfers	(1,942)		(1,942)
Comprehensive income (loss):
Net income (loss)	9,532			9,532
Other comprehensive income (loss), net of tax	273,416				273,416
Total comprehensive income (loss)	282,948
Ending Balance at Jun. 30, 2020	3,590,512	2,500	1,476,690	1,682,845	428,477
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital	75,000		75,000
Contributed (distributed) capital-parent/child asset transfers	0		0
Comprehensive income (loss):
Net income (loss)	150,078			150,078
Other comprehensive income (loss), net of tax	26,606				26,606
Total comprehensive income (loss)	176,684
Ending Balance at Sep. 30, 2020	$ 3,842,196	$ 2,500	$ 1,551,690	$ 1,832,923	$ 455,083

[1] Includes the impact from the adoption of ASU 2017-08 and 2017-12. See Note 2 to the Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 for additional information.

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