Investments (Mortgage Loans by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,572,563	$ 4,413,171
Loans Receivable, Gross, Commercial, Construction	$ 17,602	$ 9,686
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,264,623	$ 2,206,208
Loans Receivable, Gross, Commercial, Construction	$ 17,602	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	49.50%	50.00%
LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,719,540	$ 1,608,078
Loans Receivable, Gross, Commercial, Construction	$ 0	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	37.60%	36.40%
LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 441,608	$ 431,274
Loans Receivable, Gross, Commercial, Construction	$ 0	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	9.70%	9.80%
LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 146,792	$ 167,611
Loans Receivable, Gross, Commercial, Construction	$ 0	$ 9,686
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	3.20%	3.80%
Debt Service Coverage Ratio Range One [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,095,040	$ 4,182,753
Debt Service Coverage Ratio Range One [Member] | LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	2,125,328	2,148,428
Debt Service Coverage Ratio Range One [Member] | LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	1,552,568	1,517,029
Debt Service Coverage Ratio Range One [Member] | LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	315,274	396,446
Debt Service Coverage Ratio Range One [Member] | LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	101,870	120,850
Debt Service Coverage Ratio Range Two [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	258,059	151,281
Debt Service Coverage Ratio Range Two [Member] | LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	102,254	53,979
Debt Service Coverage Ratio Range Two [Member] | LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	117,751	47,128
Debt Service Coverage Ratio Range Two [Member] | LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	25,121	19,461
Debt Service Coverage Ratio Range Two [Member] | LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	12,933	30,713
Debt Service Coverage Ratio Range Three [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	201,862	69,451
Debt Service Coverage Ratio Range Three [Member] | LTV 0 to 59 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	19,439	3,801
Debt Service Coverage Ratio Range Three [Member] | LTV 60 To 69 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	49,221	43,921
Debt Service Coverage Ratio Range Three [Member] | LTV 70 To 79 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	101,213	15,367
Debt Service Coverage Ratio Range Three [Member] | LTV 80 to 100 Percent [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 31,989	$ 6,362