Employee Benefit Plans - Schedule of Aggregate Fair Value of Asset and Supplemental Asset Mix (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
CMRT asset value	$ 648.8	$ 715.5
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	54.00%	67.00%
Level 2
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	27.00%	13.00%
Level 3
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	19.00%	20.00%
Domestic Equities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	29.00%	48.00%
International Equities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	22.00%	11.00%
Fixed Income Securities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	38.00%	32.00%
Privately Managed Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	5.00%	7.00%
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	5.00%
Other, Primarily Cash
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	1.00%	2.00%