Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,364,943	$ 1,234,182
1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,331,630	1,192,274
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	29,968	32,414
Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	3,345	9,494
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	749,554	688,582
0%-59.99% | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	732,546	667,051
0%-59.99% | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	14,719	16,921
0%-59.99% | Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	2,289	4,610
60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	498,169	410,545
60%-69.99% | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	498,169	406,728
60%-69.99% | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
60%-69.99% | Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	3,817
70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	116,164	124,263
70%-79.99% | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	100,915	108,770
70%-79.99% | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	15,249	15,493
70%-79.99% | Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,056	10,792
80% or greater | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	9,725
80% or greater | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
80% or greater | Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,056	$ 1,067