Variable Interest Entities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Variable interest entities
VIE Assets	$ 51,197,981	$ 67,123,261
VIE Liabilities	50,150,781	66,130,592
Investment securities	701,818	807,618
Debt obligations to beneficial interest holders, unpaid principal balances	2,073,229	2,053,229
Interest in VIE	243,450	$ 204,605
Primary beneficiary | Accounting Standards Update 2015-02
Variable interest entities
VIE Assets	167,300
VIE Liabilities	$ 114,300
Not primary beneficiary
Variable interest entities
Number of CDO structures currently in default | item	2
Number of CDO structures that entered default during the period | item	1
Maximum risk of loss related to VIEs, on fair value basis	$ 23,800
Not primary beneficiary | Adjustment
Variable interest entities
Carrying value	0
VIE assets	(467,100)
VIE liabilities	(467,100)
Not primary beneficiary | Accounting Standards Update 2015-02 | Adjustment
Variable interest entities
Interest in VIE	118,800
Not primary beneficiary | Securitization SPEs
Variable interest entities
Debt obligations to beneficial interest holders, unpaid principal balances	$ 5,300,000