Employee Benefit Plans - Schedule of Aggregate Fair Value of Asset and Supplemental Asset Mix (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
CMRT asset value	$ 637.8	$ 648.8
CMRT asset mix	100.00%	100.00%
Total CMRT assets	100.00%	100.00%
Level 1
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	54.00%	54.00%
Level 2
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	8.00%	7.00%
Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	8.00%	9.00%
Assets Not Subject to Fair Value Hierarchy
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	30.00%	30.00%
Domestic Equities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	31.00%	29.00%
International Equities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	22.00%	22.00%
Fixed Income Securities, Principally Publicly Traded
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	36.00%	38.00%
Privately Managed Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	5.00%	5.00%
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	5.00%	5.00%
Other, Primarily Cash
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	1.00%	1.00%