Schedule IV - Mortgage Loans On Real Estate - Rollforward of mortgage loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of period	$ 3,890,751	$ 4,331,450	$ 3,861,851
Additions during period:
Originations/acquisitions	5,741,427	4,983,049	11,115,486
Net discount accretion	0	330	747
Deductions during period:
Sales	(3,982,683)	(4,509,644)	(10,071,061)
Principal repayments	(576,620)	(879,188)	(582,187)
Reversal of provision for loan losses	0	7,102	355
Transfers to REO	(4,219)	(11,566)	(5,792)
Deconsolidation adjustments	0	(6,871)	0
Changes in fair value, net	46,554	(23,911)	12,051
Balance at end of period	5,115,210	3,890,751	4,331,450
Previously Reported
Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of period	3,890,751	4,331,450	3,965,500
Deductions during period:
Balance at end of period		3,890,751	4,331,450
ASU 2014-13 election adjustment
Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of period	$ 0	0	(103,649)
Deductions during period:
Balance at end of period		$ 0	$ 0