Investments (Mortgage Loans by Maturity Date) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,413,171	$ 3,783,207
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]
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,091,066	$ 822,073
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	24.80%	21.70%
Mortgage Loans On Real Estate Due After Five Years [Member]
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,516,872	$ 2,099,559
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	57.00%	55.50%
Mortgage Loans On Real Estate Due After Ten Years [Member]
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 805,233	$ 861,575
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	18.20%	22.80%