Investments (Mortgage Loans by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 3,614,446	$ 3,136,764
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
High investment grade
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,097,202	$ 1,621,601
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	58.00%	51.70%
Investment grade
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,430,439	$ 1,397,996
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	39.60%	44.60%
Average
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 44,097	$ 87,196
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	1.20%	2.80%
Watch list
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 35,794	$ 13,550
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	1.00%	0.40%
In or near default
Mortgage Loans on Real Estate [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 6,914	$ 16,421
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	0.20%	0.50%