Variable Interest Entities and Noncontrolling Interests - Other variable interest entity disclosures (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 1,765,018	$ 1,656,189
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total VIE Assets	3,229,887	1,881,467
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(4,486)	1,752
Maximum Exposure to Loss - Off-Balance Sheet	50,140	11,190
Maximum Exposure to Loss	45,654	12,942
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2015-1 Ltd. (Jul-15)
Variable Interest Entity [Line Items]
Total VIE Assets	6,046	43,246
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	0	112
Maximum Exposure to Loss - Off-Balance Sheet	2	498
Maximum Exposure to Loss	2	610
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2017-1 Ltd. (Oct-17)
Variable Interest Entity [Line Items]
Total VIE Assets	249,737	304,373
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(345)	165
Maximum Exposure to Loss - Off-Balance Sheet	3,526	1,312
Maximum Exposure to Loss	3,181	1,477
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-1 Ltd. (Apr-18)
Variable Interest Entity [Line Items]
Total VIE Assets	362,603	374,460
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(1,351)	132
Maximum Exposure to Loss - Off-Balance Sheet	7,035	3,539
Maximum Exposure to Loss	5,684	3,671
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-2 Ltd. (Aug-18)
Variable Interest Entity [Line Items]
Total VIE Assets	507,534	653,278
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(829)	874
Maximum Exposure to Loss - Off-Balance Sheet	3,395	4,005
Maximum Exposure to Loss	2,566	4,879
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-3 Ltd. (Oct-18)
Variable Interest Entity [Line Items]
Total VIE Assets	488,430	506,110
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(916)	469
Maximum Exposure to Loss - Off-Balance Sheet	4,600	1,836
Maximum Exposure to Loss	3,684	$ 2,305
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-1 Ltd. (Mar-19)
Variable Interest Entity [Line Items]
Total VIE Assets	293,595
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(210)
Maximum Exposure to Loss - Off-Balance Sheet	4,533
Maximum Exposure to Loss	4,323
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-2 Ltd. (Apr-19)
Variable Interest Entity [Line Items]
Total VIE Assets	621,022
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(367)
Maximum Exposure to Loss - Off-Balance Sheet	15,161
Maximum Exposure to Loss	14,794
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-3 Ltd. (July-19)
Variable Interest Entity [Line Items]
Total VIE Assets	700,920
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	(468)
Maximum Exposure to Loss - Off-Balance Sheet	11,888
Maximum Exposure to Loss	$ 11,420