Income Taxes - Components of net income before income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components Of Net Income Before Income Taxes [Line Items]
Net income before income taxes	$ 640,303	$ 433,909	$ 340,334
Bermuda
Components Of Net Income Before Income Taxes [Line Items]
Net income before income taxes
Foreign - Other
Components Of Net Income Before Income Taxes [Line Items]
Net income before income taxes	$ 640,303	$ 433,909	$ 340,334