Repurchase agreements (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Secured debt [Member]	Mar. 31, 2014 Master repurchase agreement one [Member] Secured debt [Member]	Mar. 31, 2014 Master repurchase agreement three [Member] Secured debt [Member]	Mar. 31, 2014 Master repurchase agreement two [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Secured debt [Member]	Dec. 31, 2013 Variable Interest Entity, Primary Beneficiary [Member] Secured debt [Member]	Mar. 22, 2013 Variable Interest Entity, Primary Beneficiary [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement one [Member] Secured debt [Member]	Dec. 31, 2013 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement one [Member] Secured debt [Member]	Mar. 22, 2013 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement one [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Credit Suisse Repurchase Agreement [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement three [Member] Secured debt [Member]	Dec. 31, 2013 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement three [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Wells Fargo Repurchase Agreement [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement two [Member] Secured debt [Member]	Dec. 31, 2013 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement two [Member] Secured debt [Member]	Sep. 12, 2013 Variable Interest Entity, Primary Beneficiary [Member] Master repurchase agreement two [Member] Secured debt [Member]	Mar. 31, 2014 Variable Interest Entity, Primary Beneficiary [Member] Deutche Bank Repurchase Agreement [Member] Secured debt [Member]
Debt [Line Items]
Maximum borrowing capacity							$ 750,000,000	$ 750,000,000	$ 100,000,000	$ 100,000,000	$ 100,000,000	$ 100,000,000		$ 400,000,000	$ 400,000,000		$ 250,000,000	$ 250,000,000	$ 250,000,000
Interest rate			3.08%
Book value of collateral				158,576,000	678,779,000	396,916,000	1,234,271,000	1,005,912,000			166,350,000				634,234,000			205,328,000
Secured Debt, Repurchase Agreements	$ 699,950,000	$ 602,382,000						$ 602,382,000			$ 85,364,000		$ 76,800,000	$ 699,950,000	$ 398,602,000	$ 400,000,000		$ 118,416,000		$ 223,150,000