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)
Balance, beginning of period at Dec. 31, 2018			$ 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 (loss)	$ 337,687					337,687
Dividends and dividend equivalents 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	223	(901,419)	2,539,803	2,056,175	88,462
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 (loss)	166,730					166,730
Dividends and dividend equivalents 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	223	(901,419)	2,539,803	2,056,175	88,462
Balance, beginning of period at Dec. 31, 2019	4,048,723		219	(901,657)	2,449,884	2,389,789	110,488
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			2
Shares repurchased under share repurchase program (Note 13)			(11)		(226,294)
Repurchases of common stock under incentive plans				(8,081)
Issuance of common stock under incentive and benefit plans					2,271
Share-based compensation					7,088
Net income (loss)	110,510					110,510
Dividends and dividend equivalents declared						(49,876)
Net unrealized gains (losses) on investments, net of tax	101,673						101,673
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Jun. 30, 2020	3,986,005	3,986,005	210	(909,738)	2,232,949	2,450,423	212,161
Balance, beginning of period at Mar. 31, 2020			208	(902,024)	2,231,670	2,504,853	29,801
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			2
Shares repurchased under share repurchase program (Note 13)			0		0
Repurchases of common stock under incentive plans				(7,714)
Issuance of common stock under incentive and benefit plans					36
Share-based compensation					1,243
Net income (loss)	(29,951)					(29,951)
Dividends and dividend equivalents declared						(24,479)
Net unrealized gains (losses) on investments, net of tax	182,360						182,360
Net foreign currency translation adjustment, net of tax	0						0
Balance, end of period at Jun. 30, 2020	$ 3,986,005	$ 3,986,005	$ 210	$ (909,738)	$ 2,232,949	$ 2,450,423	$ 212,161