Loans Receivable - Types of Loans in Loan Portfolio, as well as Property Type and Geographic Distribution of Properties Securing these Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 3,906,226	$ 2,047,223
Senior Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	3,701,763	1,800,329
Percentage of Book Value	95.00%	88.00%
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	204,463	246,894
Percentage of Book Value	5.00%	12.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	3,906,226	2,047,223
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,698,674	864,666
Percentage of Book Value	43.00%	42.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,080,034	390,492
Percentage of Book Value	28.00%	19.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	384,188	341,819
Percentage of Book Value	10.00%	17.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	323,773	275,645
Percentage of Book Value	8.00%	13.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	190,865	43,115
Percentage of Book Value	5.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	228,692	131,486
Percentage of Book Value	6.00%	6.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,201,003	828,571
Percentage of Book Value	31.00%	40.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	668,419	469,262
Percentage of Book Value	17.00%	23.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	503,345	243,798
Percentage of Book Value	13.00%	12.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	357,777	216,429
Percentage of Book Value	9.00%	11.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	242,203	166,207
Percentage of Book Value	6.00%	8.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	322,635	85,708
Percentage of Book Value	8.00%	4.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	3,295,382	2,009,975
Percentage of Book Value	84.00%	98.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	490,558	37,248
Percentage of Book Value	13.00%	2.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	85,329
Percentage of Book Value	2.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	34,957
Percentage of Book Value	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 610,844	$ 37,248
Percentage of Book Value	16.00%	2.00%