Mortgage Loans - Allowance for Credit Losses For Mortgage Loans by Property Type (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	$ 5,125,365	$ 5,097,017
Mortgage loans on real estate	59,445	$ 19,160
Apartment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	429,600
Mortgage loans on real estate	2,555
Hotel
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	872,450
Mortgage loans on real estate	26,563
Industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	652,257
Mortgage loans on real estate	3,849
Office
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	1,565,346
Mortgage loans on real estate	5,891
Retail
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	852,213
Mortgage loans on real estate	5,650
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Mortgage loans, carrying amount	753,499
Mortgage loans on real estate	$ 14,937