Fair value of financial instruments - Fair value, assets measure on recurring basis, unobservable inputs (Details) (USD $) In Thousands, unless otherwise specified			3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Residential mortgage [Member] Fair value, inputs, level 3 [Member] Loans receivable [Member]	Mar. 31, 2013 Residential mortgage [Member] Fair value, inputs, level 3 [Member] Loans receivable [Member]
Fair value, assets measured on recurring basis, unobservable input reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Net Unrealized Gain (Loss) Included in Earnings for Loans Held			$ 38,197	$ 1,128
Mortgage Loans on Real Estate, Commercial and Consumer, Net	1,766,142	1,207,163
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance at December 31				0
Investment in mortgage loans			612,508	88,257
Net unrealized gain on mortgage loans			65,130	1,128
Net realized gain on mortgage loans			9,321	387
Mortgage loan dispositions and payments			(39,458)	(1,946)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis, Asset, Other Additions			8,006	0
Transfer of mortgage loans to real estate owned			(96,528)	(156)
Ending balance at March 31			$ 1,766,142	$ 87,670