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, 2019			$ 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		2,271
Stock Repurchased During Period, Value			(11)		(226,294)
Repurchases of common stock under incentive plans				(8,081)
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
Balance, end of period at Jun. 30, 2020		$ 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		36
Stock Repurchased During Period, Value			0		0
Repurchases of common stock under incentive plans				(7,714)
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
Balance, end of period at Jun. 30, 2020		3,986,005	210	(909,738)	2,232,949	2,450,423	212,161
Balance, beginning of period at Dec. 31, 2020	4,284,353		210	(910,115)	2,245,897	2,684,636	263,725
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1		1,937
Stock Repurchased During Period, Value			(4)		(98,691)
Repurchases of common stock under incentive plans				(10,110)
Share-based compensation					12,714
Net income (loss)	280,814					280,814
Dividends and dividend equivalents declared						(52,312)
Net unrealized gains (losses) on investments, net of tax							(84,869)
Balance, end of period at Jun. 30, 2021	4,333,833	4,333,833	207	(920,225)	2,161,857	2,913,138	178,856
Balance, beginning of period at Mar. 31, 2021			210	(910,347)	2,242,950	2,785,744	116,735
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under incentive and benefit plans			1		770
Stock Repurchased During Period, Value			(4)		(90,054)
Repurchases of common stock under incentive plans				(9,878)
Share-based compensation					8,191
Net income (loss)	155,206					155,206
Dividends and dividend equivalents declared						(27,812)
Net unrealized gains (losses) on investments, net of tax							62,121
Balance, end of period at Jun. 30, 2021	$ 4,333,833	$ 4,333,833	$ 207	$ (920,225)	$ 2,161,857	$ 2,913,138	$ 178,856