Variable Interest Entities and Noncontrolling Interests - Other variable interest entity disclosures (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 1,035,744	$ 891,774
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total VIE Assets	367,785
Maximum Exposure to Loss	2,641
Variable Interest Entity, Not Primary Beneficiary | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	547
Variable Interest Entity, Not Primary Beneficiary | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	2,094
Variable Interest Entity, Not Primary Beneficiary | Bellemeade I
Variable Interest Entity [Line Items]
Total VIE Assets	129,475
Maximum Exposure to Loss	1,654
Aggregate Excess Of Loss Reinsurance Agreement	300,000
Variable Interest Entity, Not Primary Beneficiary | Bellemeade I | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	489
Variable Interest Entity, Not Primary Beneficiary | Bellemeade I | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	1,165
Variable Interest Entity, Not Primary Beneficiary | Bellemeade II
Variable Interest Entity [Line Items]
Total VIE Assets	238,310
Maximum Exposure to Loss	987
Aggregate Excess Of Loss Reinsurance Agreement	300,000
Variable Interest Entity, Not Primary Beneficiary | Bellemeade II | On-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	58
Variable Interest Entity, Not Primary Beneficiary | Bellemeade II | Off-Balance Sheet
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	929
Variable Interest Entity, Not Primary Beneficiary | Irving Partners
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 13,900