Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,983,354	$ 4,413,171
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
Mortgage Loans On Real Estate Due One Year Through Five Years [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,425,598	$ 1,091,066
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	28.60%	24.80%
Mortgage Loans On Real Estate Due After Five Years [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,686,264	$ 2,516,872
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	53.90%	57.00%
Mortgage Loans On Real Estate Due After Ten Years [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 871,492	$ 805,233
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	17.50%	18.20%