Condensed Consolidated Statements of Changes in Common Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Parent	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update						$ (663)	$ 2,948
Balance, beginning of period at Dec. 31, 2017			$ 233	$ (893,888)	$ 2,754,275	1,116,333	23,085
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1
Shares repurchased under share repurchase program (Note 13)			(3)		(50,050)
Repurchases of common stock under incentive plans				(722)
Issuance of common stock under incentive and benefit plans					1,579
Share-based compensation					9,622
Net income	$ 323,435					323,435
Dividends declared						(1,073)
Net unrealized gains (losses) on investments, net of tax	(83,979)						(83,979)
Net foreign currency translation adjustment, net of tax	3						3
Balance, end of period at Jun. 30, 2018		$ 3,201,136	231	(894,610)	2,715,426	1,438,032	(57,943)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update						0	0
Balance, beginning of period at Mar. 31, 2018			233	(894,191)	2,748,233	1,229,616	(31,475)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1
Shares repurchased under share repurchase program (Note 13)			(3)		(40,047)
Repurchases of common stock under incentive plans				(419)
Issuance of common stock under incentive and benefit plans					146
Share-based compensation					7,094
Net income	208,949					208,949
Dividends declared						(533)
Net unrealized gains (losses) on investments, net of tax	(26,468)						(26,468)
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Jun. 30, 2018		3,201,136	231	(894,610)	2,715,426	1,438,032	(57,943)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update						0	0
Balance, beginning of period at Dec. 31, 2018	3,488,715		231	(894,870)	2,724,733	1,719,541	(60,920)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1
Shares repurchased under share repurchase program (Note 13)			(9)		(197,638)
Repurchases of common stock under incentive plans				(6,549)
Issuance of common stock under incentive and benefit plans					2,758
Share-based compensation					9,950
Net income	337,687					337,687
Dividends declared						(1,053)
Net unrealized gains (losses) on investments, net of tax	149,385						149,385
Net foreign currency translation adjustment, net of tax	(3)						(3)
Balance, end of period at Jun. 30, 2019	3,783,244	3,783,244	223	(901,419)	2,539,803	2,056,175	88,462
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update						0	0
Balance, beginning of period at Mar. 31, 2019			230	(895,321)	2,697,724	1,889,964	17,494
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1
Shares repurchased under share repurchase program (Note 13)			(8)		(165,865)
Repurchases of common stock under incentive plans				(6,098)
Issuance of common stock under incentive and benefit plans					1,689
Share-based compensation					6,255
Net income	166,730					166,730
Dividends declared						(519)
Net unrealized gains (losses) on investments, net of tax	70,971						70,971
Net foreign currency translation adjustment, net of tax	(3)						(3)
Balance, end of period at Jun. 30, 2019	$ 3,783,244	$ 3,783,244	$ 223	$ (901,419)	$ 2,539,803	$ 2,056,175	$ 88,462