Reinsurance Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Premiums Written
Written premiums, direct	$ 1,643,870	$ 1,601,344	$ 5,061,646	$ 4,852,584
Written premiums, assumed	224,979	221,237	702,265	633,200
Written premiums, ceded	(261,484)	(251,544)	(850,255)	(795,420)
Total net premiums written	1,607,365	1,571,037	4,913,656	4,690,364
Premiums Earned
Earned premiums, direct	1,647,033	1,575,624	4,824,768	4,646,343
Earned premiums, assumed	216,758	217,668	635,443	632,362
Earned premiums, ceded	(277,847)	(261,734)	(787,139)	(782,093)
Net premiums earned	1,585,944	1,531,558	4,673,072	4,496,612
Ceded losses and loss expenses incurred	213,065	123,765	507,258	374,084
Uncollectible reinsurance	1,000		1,000		$ 1,000
Fees and Commissions, Other	$ 47,315	$ 45,946	$ 143,809	$ 129,766