Income Taxes - Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of income tax expense
Current									$ 38,402	$ 43,867	$ 28,644
Deferred									38,653	59,481	50,692
Total income tax expense									77,055	103,348	79,336
Income before income taxes	$ 146,152	$ 148,431	$ 18,815	$ 176,698	$ 174,384	$ 172,225	$ 162,733	$ 149,719	490,096	659,061	546,699
Effective Income Tax Rate Reconciliation, Amount
Tax provision at weighted average statutory rates									75,763	104,213	87,815
Non-deductible expenses									2,482	2,595	1,483
Tax exempt interest, net of proration									(1,462)	(1,541)	(1,741)
Excess tax benefits from stock-based compensation									(599)	(1,997)	(9,644)
Other									871	78	1,423
Total income tax expense									$ 77,055	$ 103,348	$ 79,336
% of pretax income
Tax provision at weighted average statutory rates									15.50%	15.80%	16.10%
Non-deductible expenses									0.50%	0.40%	0.30%
Tax exempt interest, net of proration									(0.30%)	(0.20%)	(0.30%)
Excess tax benefits from stock-based compensation									(0.10%)	(0.30%)	(1.80%)
Other									0.10%	0.00%	0.20%
Total income tax expense									15.70%	15.70%	14.50%
Bermuda | Bermuda
Components of income tax expense
Income before income taxes									$ 129,300	$ 162,800	$ 128,500
US | U.S.
Components of income tax expense
Income before income taxes									$ 360,800	$ 496,300	$ 418,200