Investments (Allowance for Credit Losses and Recorded Investment) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	2,819	2,289
Total ending balance	2,819	2,289	$ 643
Recorded Investment:
Individually evaluated for impairment	7,117	1,715
Collectively evaluated for impairment	1,404,362	1,232,467
Total ending balance	1,411,479	1,234,182
Commercial Mortgage Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2,771	2,267
Total ending balance	2,771	2,267	622
Recorded Investment:
Individually evaluated for impairment	1,608	1,715
Collectively evaluated for impairment	1,326,146	1,148,232
Total ending balance	1,327,754	1,149,947
Agricultural Property Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	48	22
Total ending balance	48	22	$ 21
Recorded Investment:
Individually evaluated for impairment	5,509	0
Collectively evaluated for impairment	78,216	84,235
Total ending balance	$ 83,725	$ 84,235