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)	395,785			395,785		395,785
Other comprehensive income (loss), net of tax	176,211				176,211	176,211
Total comprehensive income (loss)						571,996
Ending Balance at Jun. 30, 2016		2,500	986,063	1,437,413	241,016	2,666,992
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	0		5,000			5,000
Dividend to parent				0		0
Contributed (distributed) capital-parent/child asset transfers			875			875
Comprehensive income (loss):
Net income (loss)	389,236			389,236		389,236
Other comprehensive income (loss), net of tax	$ 84,153				84,153	84,153
Total comprehensive income (loss)						473,389
Ending Balance at Jun. 30, 2017		$ 2,500	$ 991,937	$ 1,826,502	$ 155,128	$ 2,976,067