Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,411,479	$ 1,234,182
1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,372,964	1,192,274
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	35,679	32,414
Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	2,836	9,494
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	763,330	688,582
0%-59.99% | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	742,332	667,051
0%-59.99% | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	19,212	16,921
0%-59.99% | Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,786	4,610
60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	532,347	410,545
60%-69.99% | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	531,006	406,728
60%-69.99% | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,341	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	113,365	124,263
70%-79.99% | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	98,239	108,770
70%-79.99% | 1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	15,126	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	2,437	10,792
80% or greater | 1.2X or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,387	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	$ 1,050	$ 1,067