Fair value of financial instruments - Fair value, assets measure on recurring basis, unobservable inputs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 924,543	$ 1,853,495	$ 960,534	$ 1,959,044
Net unrealized gain on mortgage loans	2,372	42,209	28,628	103,343
Net realized gain on mortgage loans	8,180	19,272	20,912	34,654
Transfers of mortgage loans at fair value to mortgage loans held for sale, net	(137,144)	0	(103,115)	0
Mortgage loan dispositions and payments	(34,896)	(82,070)	(84,827)	(147,238)
Real estate tax advances to borrowers	1,652	4,264	5,094	11,391
Reclassification of realized gains on real estate sold from unrealized gains	0	13,175	0	23,977
Transfer of mortgage loans at fair value to real estate owned, net	(57,262)	(133,856)	(119,781)	(268,682)
Ending balance	707,445	1,716,489	707,445	1,716,489
Net unrealized gain on mortgage loans held at the end of the period	$ (10,924)	$ 29,784	$ 9,718	$ 80,852