Investments (Credit Quality Indicators) (Details) - Commercial and agricultural mortgage loans - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,389,662	$ 1,234,182
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	686,330	688,582
60% to 69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	505,251	410,545
70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	195,649	124,263
80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	2,432	10,792
1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,355,015	1,192,274
1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	667,338	667,051
1.2X | 60% to 69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	503,922	406,728
1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	182,368	108,770
1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,387	9,725
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	29,036	32,414
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	14,426	16,921
1.0X to 1.2X | 60% to 69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,329	0
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	13,281	15,493
1.0X to 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	5,611	9,494
1.0X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	4,566	4,610
1.0X | 60% to 69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	3,817
1.0X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
1.0X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,045	$ 1,067