Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,333,997	$ 3,783,207
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
Due within five years
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,086,700	$ 822,073
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	25.10%	21.70%
Due after five years through ten years
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,334,113	$ 2,099,559
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	53.80%	55.50%
Due after ten years
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 913,184	$ 861,575
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	21.10%	22.80%