Pension and Other Postretirement Benefit Plans - Textual (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Percentage of eligible pay contribution to certain employees	4.00%
Employer total contribution amount	$ 2,800,000	$ 2,500,000
Corporation match as a percentage of the participants' elective deferrals	50.00%
Maximum percent of participants' base compensation matched	6.00%
Employer contributions to Pentegra DB Plan	$ 0	0
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Contribution to defined benefit pension plan	$ 0	$ 0
Defined Benefit Pension Plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for pension plan expense	3.68%
Defined Benefit Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Liability related to SERP plan			$ 400,000