Fair value of financial instruments - Fair value, assets and liabilities measured on recurring and nonrecurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair value measurements, recurring | Fair value, inputs, level 1
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	$ 0	$ 0
Fair value measurements, recurring | Fair value, inputs, level 2
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	0	0
Fair value measurements, recurring | Fair value, inputs, level 3
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	707,445	960,534
Fair value measurements, nonrecurring | Fair value, inputs, level 1
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Real estate assets held for sale	0	0
Mortgage loans held for sale	0	0
Repurchase agreements at fair value	0	0
Other secured borrowings	0	0
Fair value measurements, nonrecurring | Fair value, inputs, level 2
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Real estate assets held for sale	0	0
Mortgage loans held for sale	0	0
Repurchase agreements at fair value	746,757	767,513
Other secured borrowings	161,496	502,268
Fair value measurements, nonrecurring | Fair value, inputs, level 3
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Real estate assets held for sale	225,682	250,557
Mortgage loans held for sale	4,058	317,336
Repurchase agreements at fair value	0	0
Other secured borrowings	$ 0	$ 0