Investments - Mortgage Loans, Residential by State (Details) - Geographic Concentration Risk - Residential mortgage loans - Mortgage Loans, Net	Sep. 30, 2019	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	100.00%	100.00%
California
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	27.90%	30.30%
Florida
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	12.80%	16.30%
TEXAS
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	5.90%	3.30%
New York
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	3.40%	7.70%
Other U.S. States
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	37.50%	42.40%
Total U.S. Region
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	87.50%	100.00%
IRELAND
Loans and Leases Receivable Disclosure [Line Items]
Percentage concentration risk	12.50%	0.00%