Income Taxes (Details) - 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 Loss Carryforwards [Line Items]
Statutory rate (as a percent)									35.00%
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
Components of income tax expense
Current									31,712	18,910	2,816
Deferred									57,564	52,157	44,614
Income tax expense									89,276	71,067	47,430
Reconciliation of the difference between income tax expense and the expected tax provision at the weighted average tax rate
Tax provision at weighted average statutory rates									91,092	72,461	47,930
Non-deductible expenses									378	381	398
Tax exempt interest, net of proration									(2,181)	(1,741)	(961)
Other									(13)	(34)	63
Income tax expense									$ 89,276	$ 71,067	$ 47,430
% of pretax income
Tax provision (benefit) at weighted average statutory rates (as a percent)									29.20%	31.70%	35.30%
Non-deductible expenses (as a percent)									0.10%	0.20%	0.30%
Tax exempt interest, net of proration (as a percent)									(0.70%)	(0.80%)	(0.70%)
Other (as a percent)									0.00%	0.00%	0.00%
Total income tax expense (as a percent)									28.60%	31.10%	34.90%
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									$ 51,600	$ 21,400
Internal Revenue Service (IRS) | U.S.
Operating Loss Carryforwards [Line Items]
Income before income taxes									$ 260,300	$ 207,000