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Pension and Other Postretirement Benefit Plans - Additional Information (Details) - Legacy Pension Plan (LPP) [Member] - Defined Benefit Pension Plan [Member] - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2015
Jun. 30, 2014
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]      
Contribution to pension plan $ 0 $ 0 $ 2,000,000
Minimum [Member]      
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]      
Employer's contribution to pension plan 0 0  
Maximum [Member]      
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]      
Employer's contribution to pension plan $ 10,000,000 $ 10,000,000