Schedule II-Condensed Financial Information of Registrant (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities
Net income	$ 62,686	$ 59,711	$ 52,258	$ 47,951	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 222,606	$ 157,331	$ 88,497
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense									16,881	13,633	12,520
Amortization of premium on investment securities									10,486	10,004	6,840
Excess tax benefits from stock-based compensation									(1,083)	(2,420)	(1,809)
Changes in assets and liabilities:
Other assets									(526)	(724)	78
Investing Activities
Net change in short-term investments									(46,352)	124,692	(196,609)
Purchase of investments available for sale									(656,768)	(798,891)	(699,324)
Proceeds from maturity of investments available for sale									27,186	8,613	27,382
Proceeds from sales of investments available for sale									313,780	449,834	144,744
Financing Activities
Revolving credit facility borrowings									100,000	0	0
Treasury stock acquired									(4,024)	(5,168)	(2,498)
Payment of issuance costs for revolving credit facility									(2,436)	0	0
Excess tax benefits from stock-based compensation									1,083	2,420	1,809
Issuance of common shares net of costs									0	537	(126,441)
Other financing activities									142	53	0
Net increase (decrease) in cash									2,925	195	(453,244)
Cash at beginning of year				24,606				24,411	24,606	24,411	477,655
Cash at end of year	27,531				24,606				27,531	24,606	24,411
Parent Company
Operating Activities
Net income									222,606	157,331	88,497
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries									(228,756)	(161,970)	(91,151)
(Gain) loss on the sale of investments, net									(111)	181	(218)
Stock-based compensation expense									880	880	1,076
Amortization of premium on investment securities									169	399	502
Excess tax benefits from stock-based compensation									(1,083)	(2,420)	(1,809)
Changes in assets and liabilities:
Other assets									121	(40)	(358)
Other accrued liabilities									17,433	15,153	13,252
Net cash provided by operating activities									11,259	9,514	9,791
Investing Activities
Net change in short-term investments									(6,525)	91,119	(114,628)
Investments in subsidiaries									(130,101)	(61,328)	(255,155)
Purchase of investments available for sale									0	(130,461)	(95,128)
Proceeds from maturity of investments available for sale									10,000	0	0
Proceeds from sales of investments available for sale									16,582	94,429	93,650
Net cash used in investing activities									(110,044)	(6,241)	(371,261)
Financing Activities
Revolving credit facility borrowings									100,000	0	0
Treasury stock acquired									(4,024)	(5,168)	(2,498)
Payment of issuance costs for revolving credit facility									(2,436)	0	0
Excess tax benefits from stock-based compensation									1,083	2,420	1,809
Issuance of common shares net of costs									0	537	(126,441)
Other financing activities									142	53	0
Net cash provided by (used in) financing activities									94,765	(3,232)	125,752
Net increase (decrease) in cash									(4,020)	41	(235,718)
Cash at beginning of year				$ 8,541				$ 8,500	8,541	8,500	244,218
Cash at end of year	$ 4,521				$ 8,541				$ 4,521	$ 8,541	$ 8,500