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)	ASU 2016-01 Retained Earnings	ASU 2016-01 Accumulated Other Comprehensive Income (Loss)	ASU 2018-02 Retained Earnings	ASU 2018-02 Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update								$ 2,061	$ 224	$ (2,724)	$ 2,724
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]
Shares repurchased under share repurchase program (Note 14)			0		(10,003)
Repurchases of common stock under incentive plans				(303)
Issuance of common stock under incentive and benefit plans					1,433
Share-based compensation					2,528
Net income	$ 114,486					114,486
Dividends declared						(540)
Net unrealized gains (losses) on investments, net of tax	(57,511)						(57,511)
Net foreign currency translation adjustment, net of tax							3
Balance, end of period at Mar. 31, 2018		$ 3,052,416	233	(894,191)	2,748,233	1,229,616	(31,475)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting standard update								$ 0	$ 0	$ 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]
Shares repurchased under share repurchase program (Note 14)			(1)		(31,773)
Repurchases of common stock under incentive plans				(451)
Issuance of common stock under incentive and benefit plans					1,069
Share-based compensation					3,695
Net income	170,957					170,957
Dividends declared						(534)
Net unrealized gains (losses) on investments, net of tax	78,414						78,414
Net foreign currency translation adjustment, net of tax							0
Balance, end of period at Mar. 31, 2019	$ 3,710,091	$ 3,710,091	$ 230	$ (895,321)	$ 2,697,724	$ 1,889,964	$ 17,494