Reserve for Losses and Loss Adjustment Expenses - Narrative and Activity in the Reserve (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) partition	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) [Abstract]
Minimum period for claims resolution (years)	5 years
Number of business partitions | partition	219
Summary of reserve for losses and loss adjustment expenses
Balance, beginning of year	$ 1,993,428	$ 2,005,326	$ 2,005,326
Less reinsurance recoverables on unpaid losses and loss adjustment expenses	273,475	249,350	249,350
Net balance, beginning of year	1,719,953	1,755,976	1,755,976
Net reserves acquired from acquisitions	0	0	0
Net losses:
Current year	147,927	139,660	587,007
Favorable development of reserves established in prior years, net	(28,776)	(28,705)	(143,778)
Total	119,151	110,955	443,229
Paid related to:
Current year	(33,085)	(26,411)	(96,190)
Prior years	(80,709)	(80,154)	(383,062)
Total paid	(113,794)	(106,565)	(479,252)
Net balance, end of year	1,725,310	1,760,367	1,719,953
Plus reinsurance recoverables on unpaid losses and loss adjustment expenses	271,679	245,403	273,475
Balance, end of year	$ 1,996,989	$ 2,005,770	$ 1,993,428