Variable interest entities and securitization activities - Assets of Unconsolidated VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying amount
Mortgage backed securities, at fair value	$ 88,011	$ 92,466
Investment in unconsolidated joint ventures	79,509	58,850
Total Assets	5,372,095	4,977,018
Unconsolidated VIEs
Carrying amount
Mortgage backed securities, at fair value	80,690	66,108
Investment in unconsolidated joint ventures	28,290	58,850
Total Assets	108,980	124,958
Maximum Exposure to Loss
Mortgage backed securities, at fair value	80,690	66,108
Investment in unconsolidated joint venture	28,290	58,850
Total assets in unconsolidated VIEs maximum exposure to loss	$ 108,980	$ 124,958