Schedule II-Financial Information of Registrant (Details 2) - 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
Revenues:
Investment income									$ 19,885	$ 12,285	$ 4,110
Realized gains (losses), net									2,554	925	116
Total revenues									353,290	239,467	131,403
Expenses:
Income before income taxes	$ 63,650	$ 59,629	$ 54,605	$ 50,514	$ 44,037	$ 38,761	$ 29,669	$ 23,460	228,398	135,927	58,027
Net income	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 28,866	$ 25,070	$ 19,555	$ 15,006	157,331	88,497	65,413
Other comprehensive income (loss):
Change in unrealized appreciation (depreciation) of investments, net of tax expense (benefit) of ($1,285) in 2015, $2,825 in 2014 and ($2,080) in 2013									(4,766)	6,114	(3,861)
Total other comprehensive income (loss)									(4,766)	6,114	(3,861)
Comprehensive income									152,565	94,611	61,552
Change in unrealized appreciation (depreciation) of investments, tax expense (benefit)									(1,285)	2,825	(2,080)
Parent Company
Revenues:
Investment income									421	1,019	4
Realized gains (losses), net									(181)	218	0
Administrative service fees from subsidiaries									437	1,020	0
Total revenues									677	2,257	4
Expenses:
Administrative service fees to subsidiaries									1,762	826	90
Other operating expenses									3,554	4,085	1,567
Total expenses									5,316	4,911	1,657
Income before income taxes									(4,639)	(2,654)	(1,653)
Loss before equity in undistributed net income of subsidiaries									(4,639)	(2,654)	(1,653)
Equity in undistributed net income of subsidiaries									161,970	91,151	67,066
Net income									157,331	88,497	65,413
Other comprehensive income (loss):
Change in unrealized appreciation (depreciation) of investments, net of tax expense (benefit) of ($1,285) in 2015, $2,825 in 2014 and ($2,080) in 2013									(4,766)	6,114	(3,861)
Total other comprehensive income (loss)									(4,766)	6,114	(3,861)
Comprehensive income									152,565	94,611	61,552
Change in unrealized appreciation (depreciation) of investments, tax expense (benefit)									$ (1,285)	$ 2,825	$ (2,080)