Pension and Other Postretirement Benefit Plans - Textual (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Percentage of eligible pay contribution to certain employees			4.00%
Employer total contribution amount	$ 2,500,000	$ 1,700,000	$ 7,500,000
Corporation match as a percentage of the participants' elective deferrals			50.00%
Maximum percent of participants' base compensation matched			4.00%
Employer contributions to Pentegra DB Plan	0	0	$ 0
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Change in funded status of plan			10,600,000
Contribution to defined benefit pension plan	0	$ 0	0	$ 0
Anticipated future employer benefit payments	5,800,000		5,800,000
Reduction in obligation due to curtailment			$ 100,000
Defined Benefit Pension Plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for pension plan expense			3.81%
Defined Benefit Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Reduction in obligation due to curtailment			$ 362,000
Liability related to SERP plan	$ 2,300,000		$ 2,300,000
Discount rate for pension plan expense			0.00%		3.63%
Corporation
Defined Benefit Plan Disclosure [Line Items]
Employer total contribution amount				$ 4,400,000