Borrowings - Repurchase agreements (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) agreement	Mar. 31, 2016 agreement	Jun. 30, 2016 USD ($) agreement	Apr. 07, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 10, 2015 USD ($)	Sep. 23, 2013 USD ($)	Mar. 22, 2013 USD ($)
Short-term Debt [Line Items]
Number of loan agreements | agreement	1		1
Deferred debt issuance costs					$ 0
Amount Outstanding	$ 740,485,000		$ 740,485,000		763,369,000
Securities Sold under Agreements to Repurchase
Short-term Debt [Line Items]
Interest rate	3.44%		3.44%
Number of repurchase agreements | agreement	2	3
Maximum Borrowing Capacity	$ 1,350,000,000		$ 1,350,000,000		1,279,944,000
Book Value of Collateral	1,265,206,000		1,265,206,000		1,378,900,000
Amount Outstanding, before debt issuance costs	746,800,000		746,800,000
Amount Outstanding	740,485,000		740,485,000		763,369,000
Amount of Available Funding	603,243,000		$ 603,243,000		512,431,000
Number of repurchase agreements, subject to certain conditions regarding collateral value | agreement			2
Securities Sold under Agreements to Repurchase | Asset-backed securities | ARLP 2015-1
Short-term Debt [Line Items]
Book Value of Collateral	34,000,000		$ 34,000,000
Securities Sold under Agreements to Repurchase | CS repurchase agreement
Short-term Debt [Line Items]
Maximum Borrowing Capacity	350,000,000		350,000,000		275,000,000			$ 100,000,000.0
Book Value of Collateral	292,104,000		292,104,000		335,184,000
Amount Outstanding, before debt issuance costs	171,837,000		171,837,000		194,346,000
Amount of Available Funding	178,163,000		178,163,000		80,654,000
Securities Sold under Agreements to Repurchase | CS repurchase agreement | ARLP 2015-1
Short-term Debt [Line Items]
Repurchase agreements	20,600,000		20,600,000
Securities Sold under Agreements to Repurchase | Wells repurchase agreement
Short-term Debt [Line Items]
Maximum Borrowing Capacity	750,000,000		750,000,000		750,000,000		$ 200,000,000.0
Book Value of Collateral	725,574,000		725,574,000		708,275,000
Amount Outstanding, before debt issuance costs	399,738,000		399,738,000		371,130,000
Amount of Available Funding	350,262,000		350,262,000		378,870,000
Securities Sold under Agreements to Repurchase | DB repurchase agreement
Short-term Debt [Line Items]
Maximum Borrowing Capacity					54,944,000
Book Value of Collateral					130,863,000
Amount Outstanding, before debt issuance costs					54,944,000
Amount of Available Funding					0
Securities Sold under Agreements to Repurchase | Nomura loan and security agreement
Short-term Debt [Line Items]
Maximum Borrowing Capacity				$ 250,000,000.0		$ 100,000,000.0
Securities Sold under Agreements to Repurchase and Loan Agreements [Member]
Short-term Debt [Line Items]
Deferred debt issuance costs	(6,272,000)		(6,272,000)		(4,144,000)
Loan and Security Agreement | Nomura loan and security agreement
Short-term Debt [Line Items]
Maximum Borrowing Capacity	250,000,000		250,000,000		200,000,000
Book Value of Collateral	247,528,000		247,528,000		204,578,000
Amount Outstanding, before debt issuance costs	175,182,000		175,182,000		147,093,000
Amount of Available Funding	$ 74,818,000		$ 74,818,000		$ 52,907,000