Consolidated Statements of Stockholder's Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Total Equity
Beginning Balance at Dec. 31, 2012		$ 2,500	$ 818,303	$ 2,427,628	$ 267,461	$ 3,515,892
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(423,000)		(423,000)
Contributed (distributed) capital-parent/child asset transfers			(14,066)			(14,066)
Comprehensive income:
Net income	$ 1,538,210			1,538,210		1,538,210
Other comprehensive income (loss), net of tax	(210,815)				(210,815)	(210,815)
Total comprehensive income						1,327,395
Ending Balance at Dec. 31, 2013		2,500	804,237	3,542,838	56,646	4,406,221
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(748,000)		(748,000)
Contributed (distributed) capital-parent/child asset transfers			(12,084)			(12,084)
Comprehensive income:
Net income	785,803			785,803		785,803
Other comprehensive income (loss), net of tax	122,045				122,045	122,045
Total comprehensive income						907,848
Ending Balance at Dec. 31, 2014		2,500	792,153	3,580,641	178,691	4,553,985
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(430,000)		(430,000)
Contributed (distributed) capital-parent/child asset transfers			(12,180)			(12,180)
Comprehensive income:
Net income	512,898			512,898		512,898
Other comprehensive income (loss), net of tax	$ (113,886)				(113,886)	(113,886)
Total comprehensive income						399,012
Ending Balance at Dec. 31, 2015		$ 2,500	$ 779,973	$ 3,663,539	$ 64,805	$ 4,510,817