Investments (Mortgage Loans by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 5,134,898	$ 4,983,354
Loans Receivable, Gross, Commercial, Construction	$ 37,607	$ 32,620
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	100.00%	100.00%
Mortgage Loans By Credit Quality Indicator Range Four [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 209,424	$ 193,206
Loans Receivable, Gross, Commercial, Construction	$ 0	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	4.10%	3.90%
Mortgage Loans By Credit Quality Indicator Range Three [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 472,772	$ 517,147
Loans Receivable, Gross, Commercial, Construction	$ 0	$ 0
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	9.20%	10.40%
Mortgage Loans By Credit Quality Indicator Range Two [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 1,833,839	$ 1,749,331
Loans Receivable, Gross, Commercial, Construction	$ 21,051	$ 18,929
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	35.70%	35.10%
Mortgage Loans By Credit Quality Indicator Range One [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 2,618,863	$ 2,523,670
Loans Receivable, Gross, Commercial, Construction	$ 16,556	$ 13,691
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total	51.00%	50.60%
Debt Service Coverage Ratio Range Three [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 163,198	$ 156,297
Debt Service Coverage Ratio Range Three [Member] | Mortgage Loans By Credit Quality Indicator Range Four [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	42,512	25,560
Debt Service Coverage Ratio Range Three [Member] | Mortgage Loans By Credit Quality Indicator Range Three [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	42,008	54,440
Debt Service Coverage Ratio Range Three [Member] | Mortgage Loans By Credit Quality Indicator Range Two [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	41,680	38,120
Debt Service Coverage Ratio Range Three [Member] | Mortgage Loans By Credit Quality Indicator Range One [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	36,998	38,177
Debt Service Coverage Ratio Range Two [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	243,474	233,260
Debt Service Coverage Ratio Range Two [Member] | Mortgage Loans By Credit Quality Indicator Range Four [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	49,522	49,668
Debt Service Coverage Ratio Range Two [Member] | Mortgage Loans By Credit Quality Indicator Range Three [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	27,591	48,438
Debt Service Coverage Ratio Range Two [Member] | Mortgage Loans By Credit Quality Indicator Range Two [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	78,559	73,908
Debt Service Coverage Ratio Range Two [Member] | Mortgage Loans By Credit Quality Indicator Range One [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	87,802	61,246
Debt Service Coverage Ratio Range One [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	4,690,619	4,561,177
Debt Service Coverage Ratio Range One [Member] | Mortgage Loans By Credit Quality Indicator Range Four [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	117,390	117,978
Debt Service Coverage Ratio Range One [Member] | Mortgage Loans By Credit Quality Indicator Range Three [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	403,173	414,269
Debt Service Coverage Ratio Range One [Member] | Mortgage Loans By Credit Quality Indicator Range Two [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	1,692,549	1,618,374
Debt Service Coverage Ratio Range One [Member] | Mortgage Loans By Credit Quality Indicator Range One [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Financing Receivable, Gross	$ 2,477,507	$ 2,410,556