Pension and Postretirement Benefit Plans Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Multiemployer Plans [Line Items]
Defined Contribution Plan, Cost	$ 268,000	$ 237,000	$ 721,000	$ 624,000
Pentegra Defined Benefit Plan [Member]
Multiemployer Plans [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 1,644,000	$ 1,685,000	$ 3,289,000	$ 3,370,000