Reserve for Losses and Loss Adjustment Expenses - Narrative and Activity in the Reserve (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($) partition	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) partition	Dec. 31, 2015 USD ($)	Dec. 31, 2014 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								219
Summary of reserve for losses and loss adjustment expenses
Balance, beginning of year				$ 2,005,326				$ 2,058,266	$ 2,005,326	$ 2,058,266	$ 2,072,822
Less reinsurance recoverables on unpaid losses and loss adjustment expenses				249,350				237,966	249,350	237,966	247,518
Net balance, beginning of year				1,755,976				1,820,300	1,755,976	1,820,300	1,825,304
Net reserves acquired from acquisitions									0	0	139,549
Net losses:
Current year	$ 156,585	$ 147,093	$ 143,668	139,660	$ 149,157	$ 145,027	$ 139,057	138,654	587,007	571,891	545,168
Favorable development of reserves established in prior years, net	(49,292)	$ (29,011)	$ (36,769)	$ (28,705)	(56,330)	$ (36,221)	$ (35,115)	$ (33,514)	(143,778)	(161,180)	(182,084)
Total									443,229	410,711	363,084
Paid related to:
Current year									(96,190)	(84,186)	(93,737)
Prior years									(383,062)	(390,849)	(413,900)
Total paid									(479,252)	(475,035)	(507,637)
Net balance, end of year	1,719,953				1,755,976				1,719,953	1,755,976	1,820,300
Plus reinsurance recoverables on unpaid losses and loss adjustment expenses	273,475				249,350				273,475	249,350	237,966
Balance, end of year	$ 1,993,428				$ 2,005,326				$ 1,993,428	$ 2,005,326	$ 2,058,266