Income Taxes (Details) - 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 Loss Carryforwards [Line Items]
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
Statutory rate (as a percent)									35.00%
Components of income tax expense
Current									$ 18,910	2,816	881
Deferred									52,157	44,614	(8,267)
Total income tax expense (benefit)									71,067	47,430	(7,386)
Reconciliation of the difference between income tax expense and the expected tax provision at the weighted average tax rate
Tax provision (benefit) at weighted average statutory rates									72,461	47,930	21,405
Non-deductible expenses									381	398	276
Tax exempt interest, net of proration									(1,741)	(961)	(159)
Change in valuation allowance									0	0	(28,886)
Other									(34)	63	(22)
Total income tax expense (benefit)									$ 71,067	$ 47,430	$ (7,386)
% of pretax income
Tax provision (benefit) at weighted average statutory rates (as a percent)									31.70%	35.30%	36.90%
Non-deductible expenses (as a percent)									0.20%	0.30%	0.50%
Tax exempt interest, net of proration (as a percent)									(0.80%)	(0.70%)	(0.30%)
Change in valuation allowance (as a percent)									0.00%	0.00%	(49.80%)
Other (as a percent)									0.00%	0.00%	0.00%
Total income tax expense (as a percent)									31.10%	34.90%	(12.70%)
Bermuda | Bermuda
Operating Loss Carryforwards [Line Items]
Statutory rate (as a percent)									0.00%
Office of the Tax Commissioner, Bermuda | Bermuda
Operating Loss Carryforwards [Line Items]
Income before income taxes									$ 21,400
Internal Revenue Service (IRS) [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Income before income taxes									$ 207,000