Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	Jun. 30, 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	$ 4,572,563	$ 4,413,171
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]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,153,623	$ 1,091,066
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	25.20%	24.80%
Due after five years through ten years
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,623,105	$ 2,516,872
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	57.40%	57.00%
Due after ten years
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 795,835	$ 805,233
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	17.40%	18.20%