Employee Benefit Plans - Schedule of Aggregate Fair Value of Asset and Supplemental Asset Mix (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
CMRT asset value	$ 715.5	$ 722.8
CMRT fair value input	100.00%	100.00%
CMRT asset mix	100.00%	100.00%
Level 1
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	67.00%	79.00%
Level 2
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	13.00%	4.00%
Level 3
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	20.00%	17.00%
Domestic Equities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	48.00%	53.00%
International Equities, Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	11.00%
Fixed Income Securities, Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	32.00%	35.00%
Privately Managed Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	7.00%	11.00%
Other, Primarily Cash
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	2.00%	1.00%