Schedule II-Financial Information of Registrant (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities
Net income	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 28,866	$ 25,070	$ 19,555	$ 15,006	$ 157,331	$ 88,497	$ 65,413
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on the sale of investments, net									(2,554)	(925)	(116)
Stock-based compensation expense									13,633	12,520	3,597
Amortization of premium on investment securities									10,004	6,840	3,129
Excess tax benefits from stock-based compensation									(2,420)	(1,809)	0
Changes in assets and liabilities:
Other assets									(724)	78	(1,628)
Investing Activities
Net change in short-term investments									124,692	(196,609)	(6,982)
Purchase of investments available for sale									(798,891)	(699,324)	(136,094)
Proceeds from sales of investments available for sale									449,834	144,744	36,582
Financing Activities
Issuance of common shares net of costs									(537)	126,441	438,403
Treasury stock acquired									(5,168)	(2,498)	(311)
Excess tax benefits from stock-based compensation									2,420	1,809	0
Other financing activities									53	0	0
Net increase (decrease) in cash									195	(453,244)	455,340
Cash at beginning of year				24,411				477,655	24,411	477,655	22,315
Cash at end of year	24,606				24,411				24,606	24,411	477,655
Parent Company
Operating Activities
Net income									157,331	88,497	65,413
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries									(161,970)	(91,151)	(67,066)
Gain on the sale of investments, net									181	(218)	0
Stock-based compensation expense									880	1,076	0
Amortization of premium on investment securities									399	502	0
Excess tax benefits from stock-based compensation									(2,420)	(1,809)	0
Changes in assets and liabilities:
Other assets									(40)	(358)	(1,450)
Other accrued liabilities									15,153	13,252	4,196
Net cash provided by operating activities									9,514	9,791	1,093
Investing Activities
Net change in short-term investments									91,119	(114,628)	999
Investments in subsidiaries									(61,328)	(255,155)	(197,000)
Purchase of investments available for sale									(130,461)	(95,128)	0
Proceeds from sales of investments available for sale									94,429	93,650	0
Net cash used in investing activities									(6,241)	(371,261)	(196,001)
Financing Activities
Issuance of common shares net of costs									(537)	126,441	438,403
Treasury stock acquired									(5,168)	(2,498)	(311)
Excess tax benefits from stock-based compensation									2,420	1,809	0
Other financing activities									53	0	0
Net cash provided by financing activities									(3,232)	125,752	438,092
Net increase (decrease) in cash									41	(235,718)	243,184
Cash at beginning of year				$ 8,500				$ 244,218	8,500	244,218	1,034
Cash at end of year	$ 8,541				$ 8,500				$ 8,541	$ 8,500	$ 244,218