Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 5,134,898	$ 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, Gross	$ 1,449,925	$ 1,425,598
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	28.20%	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, Gross	$ 2,833,275	$ 2,686,264
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	55.20%	53.90%
Due after ten years
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 851,698	$ 871,492
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	16.60%	17.50%