Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,355,375	$ 1,389,662
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	673,788	686,330
60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	501,871	505,251
70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	176,690	195,649
80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	3,026	2,432
≥ 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,308,520	1,355,015
≥ 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	658,709	667,338
≥ 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	478,402	503,922
≥ 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	169,409	182,368
≥ 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	2,000	1,387
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	46,855	29,036
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	15,079	14,426
1.0X to 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	23,469	1,329
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	7,281	13,281
1.0X to 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,026	0
Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	5,611
Less than 1.0X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	4,566
Less than 1.0X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
Less than 1.0X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
Less than 1.0X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 0	$ 1,045