Employee Benefits - Weighted Average Assumptions Used to Determine Net Pension and Other Postretirement Benefit Costs (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.20%	4.40%	5.00%
Rate of compensation increase	4.50%	4.50%	4.50%
Expected long term rate of return on plan assets	7.50%	7.50%	7.75%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.20%	4.40%	5.00%
Rate of compensation increase
Expected long term rate of return on plan assets	7.50%	7.50%	7.75%