Investments (Mortgage Loans by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,413,171	$ 3,783,207
Loans Receivable, Gross, Commercial, Construction	$ 9,686
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
Greater than 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 4,182,753	$ 3,601,817
1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	151,281	120,296
Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	69,451	61,094
0% - 59.99%
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	2,206,208	$ 1,925,755
Loans Receivable, Gross, Commercial, Construction	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	50.00%	50.80%
0% - 59.99% | Greater than 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,148,428	$ 1,859,640
0% - 59.99% | 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	53,979	64,749
0% - 59.99% | Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	3,801	1,366
60% - 69.99%
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	1,608,078	$ 1,292,466
Loans Receivable, Gross, Commercial, Construction	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	36.40%	34.20%
60% - 69.99% | Greater than 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 1,517,029	$ 1,257,788
60% - 69.99% | 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	47,128	34,678
60% - 69.99% | Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	43,921	0
70% - 79.99%
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	431,274	$ 415,330
Loans Receivable, Gross, Commercial, Construction	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	9.80%	11.00%
70% - 79.99% | Greater than 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 396,446	$ 370,092
70% - 79.99% | 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	19,461	20,869
70% - 79.99% | Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	15,367	24,369
Greater than 80%
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	167,611	$ 149,656
Loans Receivable, Gross, Commercial, Construction	$ 9,686
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	3.80%	4.00%
Greater than 80% | Greater than 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 120,850	$ 114,297
Greater than 80% | 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	30,713	0
Greater than 80% | Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 6,362	$ 35,359