Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Document Period End Date	Sep. 30, 2019
Financing Receivable, before Allowance for Credit Loss	$ 5,667,354	$ 4,983,354
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
Due within five years
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 1,750,771	$ 1,425,598
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	30.90%	28.60%
Due after five years through ten years
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 3,001,527	$ 2,686,264
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	53.00%	53.90%
Due after ten years
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 915,056	$ 871,492
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	16.10%	17.50%