Employee benefit plans - Schedule of Aggregate Fair Value of Asset and Supplemental Asset Mix (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
CMRT asset value	$ 726.4	$ 659.5
CMRT fair value input	100.00%	100.00%
CMRT asset mix	100.00%	100.00%
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	82.00%	82.00%
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	1.00%	1.00%
Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT fair value input	17.00%	17.00%
Domestic Equities, Principally Publicly Traded [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	43.00%	75.00%
International Equities, Publicly Traded [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	2.00%	2.00%
Fixed Income Securities, Publicly Traded [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	12.00%	14.00%
Privately Managed Limited Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	8.00%	8.00%
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
CMRT asset mix	35.00%	1.00%