Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Written premiums:
Written premiums, direct	$ 1,853,881	$ 1,746,197	$ 5,566,728	$ 5,318,698
Written premiums, assumed	239,169	181,587	662,412	536,582
Written premiums, ceded	(343,144)	(303,570)	(1,026,169)	(941,624)
Total net premiums written	1,749,906	1,624,214	5,202,971	4,913,656
Earned premiums:
Earned premiums, direct	1,797,726	1,735,576	5,294,773	5,115,403
Earned premiums, assumed	209,094	177,996	595,946	547,372
Earned premiums, ceded	(330,010)	(310,087)	(974,212)	(910,828)
Net premiums earned	1,676,810	1,603,485	4,916,507	4,751,947
Ceded losses and loss expenses incurred	172,009	186,538	584,323	586,607
Ceded commissions earned	76,001	$ 68,938	220,110	$ 201,216
Uncollectible reinsurance	$ 1,000		$ 1,000		$ 1,000