Concentration of Credit Risk - Summary of Loan Portfolio by Property Type (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 4,705,831	$ 3,727,156
Unfunded Commitment	$ 527,102	$ 529,040
% of Loan Commitment	100.00%	100.00%
Loan UPB	$ 4,178,729	$ 3,198,116
% of Loan UPB	100.00%	100.00%
Condominium
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 276,044	$ 679,779
Unfunded Commitment	$ 25,462	$ 166,358
% of Loan Commitment	5.90%	18.20%
Loan UPB	$ 250,582	$ 513,421
% of Loan UPB	6.00%	16.10%
Multifamily
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 950,808	$ 813,775
Unfunded Commitment	$ 76,023	$ 75,509
% of Loan Commitment	20.20%	21.80%
Loan UPB	$ 874,785	$ 738,266
% of Loan UPB	20.90%	23.10%
Office
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,776,866	$ 836,826
Unfunded Commitment	$ 272,336	$ 160,450
% of Loan Commitment	37.80%	22.50%
Loan UPB	$ 1,504,530	$ 676,376
% of Loan UPB	36.00%	21.10%
Hotel
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 627,994	$ 693,569
Unfunded Commitment	$ 13,290	$ 27,980
% of Loan Commitment	13.30%	18.60%
Loan UPB	$ 614,704	$ 665,589
% of Loan UPB	14.70%	20.80%
Mixed Use
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 758,500	$ 431,500
Unfunded Commitment	$ 87,567	$ 57,243
% of Loan Commitment	16.10%	11.60%
Loan UPB	$ 670,933	$ 374,257
% of Loan UPB	16.10%	11.70%
Retail
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 239,058	$ 195,012
Unfunded Commitment	$ 52,424	$ 41,500
% of Loan Commitment	5.10%	5.20%
Loan UPB	$ 186,634	$ 153,512
% of Loan UPB	4.50%	4.80%
Industrial
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 66,500	$ 66,500
% of Loan Commitment	1.40%	1.80%
Loan UPB	$ 66,500	$ 66,500
% of Loan UPB	1.60%	2.10%
Other
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 10,061	$ 10,195
% of Loan Commitment	0.20%	0.30%
Loan UPB	$ 10,061	$ 10,195
% of Loan UPB	0.20%	0.30%