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, 2015			$ 2,500	$ 779,973	$ 3,635,147	$ 64,805	$ 4,482,425
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital	$ 405,321			205,000			205,000
Dividend to parent					(2,593,519)		(2,593,519)
Contributed (distributed) capital-parent/child asset transfers				1,090			1,090
Comprehensive income (loss):
Net income (loss)	480,682	[1]			480,682		480,682
Other comprehensive income (loss), net of tax	184,982					184,982	184,982
Total comprehensive income (loss)							665,664
Ending Balance at Sep. 30, 2016			2,500	986,063	1,522,310	249,787	2,760,660
Beginning Balance at Dec. 31, 2016			2,500	986,062	1,437,266	70,975	2,496,803
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital	148,500			153,500			153,500
Dividend to parent					0		0
Contributed (distributed) capital-parent/child asset transfers				875			875
Comprehensive income (loss):
Net income (loss)	210,740	[1]			210,740		210,740
Other comprehensive income (loss), net of tax	$ 77,698					77,698	77,698
Total comprehensive income (loss)							288,438
Ending Balance at Sep. 30, 2017			$ 2,500	$ 1,140,437	$ 1,648,006	$ 148,673	$ 2,939,616

[1] Prior period amounts for the three months ended September 30, 2016 have been revised to reflect a corrected tax expense amount.