Pensions and Other Postretirement Benefits - Aggregate Fair Value of CMRT Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Combined Master Retirement Trust Asset [Line Items]
CMRT asset value	$ 731.0	$ 666.6
CMRT [Member]
CMRT fair value input:
Fair value input	100.00%	100.00%
CMRT asset mix:
Percentage of combined master retirement trust asset mix	100.00%	100.00%
CMRT [Member] | Domestic equities, principally publically traded [Member]
CMRT asset mix:
Percentage of combined master retirement trust asset mix	43.00%	75.00%
CMRT [Member] | International equities, publically traded [Member]
CMRT asset mix:
Percentage of combined master retirement trust asset mix	2.00%	2.00%
CMRT [Member] | Fixed income securities, publically traded [Member]
CMRT asset mix:
Percentage of combined master retirement trust asset mix	11.00%	14.00%
CMRT [Member] | Privately managed limited partnerships [Member]
CMRT asset mix:
Percentage of combined master retirement trust asset mix	8.00%	8.00%
CMRT [Member] | Other [Member]
CMRT asset mix:
Percentage of combined master retirement trust asset mix	36.00%	1.00%
CMRT [Member] | Level 1 [Member]
CMRT fair value input:
Fair value input	82.00%	82.00%
CMRT [Member] | Level 2 [Member]
CMRT fair value input:
Fair value input	1.00%	1.00%
CMRT [Member] | Level 3 [Member]
CMRT fair value input:
Fair value input	17.00%	17.00%