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Pension and Other Postretirement Benefit Plans - Textual (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 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 $ 2,000,000 $ 2,400,000 $ 4,800,000 $ 5,000,000
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     3.68%  
Defined Benefit Pension Plans | Nonqualified Plan        
Defined Benefit Plan Disclosure [Line Items]        
Liability related to SERP plan $ 400,000   $ 400,000