Schedule II-Condensed Financial Information of Registrant (Details 2) - 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
Condensed Statements of Comprehensive Income (Loss)
Issuance of common shares net of costs									$ 0	$ (537)	$ 126,441
Revenues:
Net investment income									27,890	19,885	12,285
Total revenues									458,258	353,290	239,467
Expenses:
Income before income taxes	$ 87,302	$ 83,441	$ 73,792	$ 67,347	$ 63,650	$ 59,629	$ 54,605	$ 50,514	311,882	228,398	135,927
Net income	$ 62,686	$ 59,711	$ 52,258	$ 47,951	$ 44,479	$ 40,821	$ 37,193	$ 34,838	222,606	157,331	88,497
Other comprehensive income (loss):
Change in unrealized (depreciation) appreciation of investments, net of tax (benefit) expense of ($2,941) in 2016, ($1,285) in 2015 and $2,825 in 2014									(12,156)	(4,766)	6,114
Change in unrealized appreciation (depreciation) of investments, tax expense (benefit)									(2,941)	(1,285)	2,825
Total other comprehensive (loss) income									(12,156)	(4,766)	6,114
Comprehensive income									210,450	152,565	94,611
Parent Company
Condensed Statements of Comprehensive Income (Loss)
Issuance of common shares net of costs									0	(537)	126,441
Revenues:
Net investment income									284	421	1,019
Realized investment gains (losses), net									111	(181)	218
Administrative service fees from subsidiaries									473	437	1,020
Total revenues									868	677	2,257
Expenses:
Administrative service fees to subsidiaries									1,760	1,762	826
Other operating expenses									5,258	3,554	4,085
Total expenses									7,018	5,316	4,911
Income before income taxes									(6,150)	(4,639)	(2,654)
Loss before equity in undistributed net income of subsidiaries									(6,150)	(4,639)	(2,654)
Equity in undistributed net income of subsidiaries									228,756	161,970	91,151
Net income									222,606	157,331	88,497
Other comprehensive income (loss):
Change in unrealized (depreciation) appreciation of investments, net of tax (benefit) expense of ($2,941) in 2016, ($1,285) in 2015 and $2,825 in 2014									(12,156)	(4,766)	6,114
Change in unrealized appreciation (depreciation) of investments, tax expense (benefit)									(2,941)	(1,285)	2,825
Total other comprehensive (loss) income									(12,156)	(4,766)	6,114
Comprehensive income									$ 210,450	$ 152,565	$ 94,611