Borrowings - Repurchase Agreements Narrative (Details) - CS repurchase agreement - USD ($)	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Mar. 22, 2013
Repurchase agreements
Short-term Debt [Line Items]
Maximum borrowing capacity			$ 600,000,000.0	$ 100,000,000.0
Repurchase agreements | Forecast
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 350,000,000
Repurchase agreements
Short-term Debt [Line Items]
Maximum borrowing capacity		$ 450,000,000	600,000,000
Amount Outstanding, before debt issuance costs		$ 271,833,000	$ 582,659,000