Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	2,278	2,289
Total ending balance	2,278	2,289
Recorded Investment:
Individually evaluated for impairment	1,680	1,715
Collectively evaluated for impairment	1,275,682	1,232,467
Total ending balance	1,277,362	1,234,182
Commercial Mortgage Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2,243	2,267
Total ending balance	2,243	2,267
Recorded Investment:
Individually evaluated for impairment	1,680	1,715
Collectively evaluated for impairment	1,187,750	1,148,232
Total ending balance	1,189,430	1,149,947
Agricultural Property Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	35	22
Total ending balance	35	22
Recorded Investment:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	87,932	84,235
Total ending balance	$ 87,932	$ 84,235