Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,379,635	$ 1,389,662
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	631,552	686,330
60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	544,970	505,251
70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	202,080	195,649
80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,033	2,432
≥ 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,329,676	1,355,015
≥ 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	613,628	667,338
≥ 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	525,861	503,922
≥ 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	190,187	182,368
≥ 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	1,387
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	49,959	29,036
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	17,924	14,426
1.0X to 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	19,109	1,329
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	11,893	13,281
1.0X to 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,033	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