Pension and Other Postretirement Benefit Plans - Textual (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Percentage of eligible pay contribution to certain employees			4.00%
Employer total contribution amount	$ 2,400,000	$ 1,400,000	$ 5,000,000
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]
Contribution to defined benefit pension plan	$ 0	$ 0	$ 0	$ 0
Defined Benefit Pension Plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for pension plan expense			4.22%
Defined Benefit Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for pension plan expense			3.63%
Postretirement Benefit Plan | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for pension plan expense			3.79%
Corporation
Defined Benefit Plan Disclosure [Line Items]
Employer total contribution amount				$ 2,700,000