Reserve for Losses and Loss Adjustment Expenses - Reconciliation of Claims Development (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	$ 1,541,464
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	273,475	$ 249,350	$ 237,966	$ 247,518
Reserve for the future utilization of the DDR benefit	74,200
Unallocated loss adjustment expenses	111,628
Purchase accounting	4,653
Other	(11,992)
Liability for unpaid claims and claim adjustment expense	178,489
Gross liability for losses and loss adjustment expenses	1,993,428	$ 2,005,326	$ 2,058,266	$ 2,072,822
Healthcare professional liability claims-made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	987,192
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	132,460
Healthcare professional liability occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	183,221
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	22,576
Medical technology liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	45,951
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	44,627
Workers' compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	180,372
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	52,111
Lloyd's syndicate casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	27,240
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	172
Lloyd's syndicate property insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	6,038
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	3,074
Lloyd's syndicate property reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	4,951
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	2,094
Other short-duration lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	106,499
Total reinsurance recoverable on unpaid losses and loss adjustment expenses	$ 16,361