Borrowings - Repurchase and loan Agreements narrative (Details)	Sep. 30, 2017 USD ($) agreement	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Number of repurchase agreements at period end | agreement	1
Number of loan agreements at period end | agreement	5
Aggregate outstanding debt under repurchase and loan agreements	$ 1,100,106,000	$ 1,220,972,000
Repurchase and loan agreements
Debt Instrument [Line Items]
Interest rate on debt	4.45%
Maximum aggregate funding available under repurchase and loan agreements	$ 1,356,941,000	1,339,259,000
Aggregate outstanding debt under repurchase and loan agreements	$ 1,100,106,000	$ 1,220,972,000