Variable interest entities and securitization activities - Assets of Unconsolidated VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Carrying amount
Mortgage backed securities, at fair value	$ 682,948	$ 88,011
Investment in unconsolidated joint ventures	75,048	79,509
Total Assets	8,016,955	5,372,095
Unconsolidated VIEs
Carrying amount
Mortgage backed securities, at fair value	82,158	80,690
Investment in unconsolidated joint ventures	26,298	28,290
Total Assets	108,456	108,980
Maximum Exposure to Loss
Mortgage backed securities, at fair value	82,158	80,690
Investment in unconsolidated joint venture	26,298	28,290
Total assets in unconsolidated VIEs maximum exposure to loss	$ 108,456	$ 108,980