Schedule VI - Supplemental information for property-casualty underwriters (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Losses and loss expenses incurred related to current year	$ 1,281,289	$ 1,283,970	$ 1,024,256
Losses and loss expenses incurred related to prior year	(216,192)	(306,137)	(259,241)
Property and casualty insurance company subsidiaries
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred acquisition costs	209,227	181,002	161,022
Reserves for losses and loss expenses	2,995,195	2,996,567	3,243,147
Reserves for unearned premiums	1,076,049	966,210	989,229
Net earned premiums	2,249,165	2,246,889	1,993,055
Net investment income	150,385	127,824	100,086
Losses and loss expenses incurred related to current year	1,281,289	1,283,970	1,024,256
Losses and loss expenses incurred related to prior year	(216,192)	(306,137)	(259,241)
Net paid losses and loss expenses	1,114,402	1,167,839	1,063,653
Amortization of deferred acquisition costs	449,482	410,058	339,467
Net premiums written	$ 2,359,000	$ 2,228,825	$ 2,045,657