Commercial Mortgage Loans - Allowance Past Due (Details) - Commercial Portfolio Segment - Financing Receivable, Held-to-Maturity $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Financing Receivable, Past Due [Line Items]
Current	$ 2,560,347
Total	2,662,681
1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	45,259
Interest income recognized	600
30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
120 days past due
Financing Receivable, Past Due [Line Items]
Past due	57,075
Interest income recognized	0
Multifamily
Financing Receivable, Past Due [Line Items]
Current	1,392,741
Total	1,392,741
Multifamily | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Multifamily | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Multifamily | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Multifamily | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Multifamily | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Retail
Financing Receivable, Past Due [Line Items]
Current	127,795
Total	127,795
Retail | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Retail | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Retail | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Retail | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Retail | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Office
Financing Receivable, Past Due [Line Items]
Current	383,938
Total	429,197
Office | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	45,259
Office | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Office | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Office | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Office | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial
Financing Receivable, Past Due [Line Items]
Current	143,472
Total	143,472
Industrial | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Mixed Use
Financing Receivable, Past Due [Line Items]
Current	65,050
Total	65,050
Mixed Use | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Mixed Use | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Mixed Use | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Mixed Use | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Mixed Use | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial
Financing Receivable, Past Due [Line Items]
Current	339,700
Total	396,775
Industrial | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Industrial | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	57,075
Self Storage
Financing Receivable, Past Due [Line Items]
Current	62,937
Total	62,937
Self Storage | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Self Storage | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Self Storage | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Self Storage | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Self Storage | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Manufactured Housing
Financing Receivable, Past Due [Line Items]
Current	44,714
Total	44,714
Manufactured Housing | 1-29 Days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Manufactured Housing | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Manufactured Housing | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Manufactured Housing | 90-119 days past due
Financing Receivable, Past Due [Line Items]
Past due	0
Manufactured Housing | 120 days past due
Financing Receivable, Past Due [Line Items]
Past due	$ 0