Reinsurance Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Premiums Written
Written premiums, direct	$ 1,709,129	$ 1,647,341
Written premiums, assumed	246,568	204,464
Written premiums, ceded	(291,975)	(276,403)
Total net premiums written	1,663,722	1,575,402
Premiums Earned
Earned premiums, direct	1,568,067	1,524,736
Earned premiums, assumed	218,367	203,342
Earned premiums, ceded	(259,099)	(256,065)
Net premiums earned	1,527,335	1,472,013	$ 6,040,609	$ 5,744,418	$ 5,226,537
Ceded losses and loss expenses incurred	126,929	118,391
Uncollectible reinsurance	1,000		$ 1,000
Fees and Commissions, Other	$ 47,973	$ 43,651