Pension and Other Postretirement Benefit Plans - Textual (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Maximum percent of participants' base compensation matched			6.00%
Percentage of eligible pay contribution to certain employees			4.00%
Corporation match as a percentage of the participants' elective deferrals			50.00%
Employer total contribution amount	$ 1,500,000	$ 2,500,000	$ 6,300,000	$ 7,500,000
Employer contributions to Pentegra DB Plan	0	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			3.68%
Defined Benefit Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Liability related to SERP plan	$ 400,000		$ 400,000