Summary of Total Indebtedness (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Fair value of interest rate swap	$ 24.8	$ 20.5
Series Two Thousand Five Dash One Notes [Member]
Debt Instrument [Line Items]
Fair value of interest rate swap		10.3
Twenty Ten Senior Notes [Member]
Debt Instrument [Line Items]
Fair value of interest rate swap	2.1	0
Unamortized discount	2.0	2.2
Twenty Twelve Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	3.2	3.5
Twenty Thirteen Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	2.6	2.8
Two Thousand Fourteen Senior Notes Five Year [Member]
Debt Instrument [Line Items]
Fair value of interest rate swap	1.3	0
Unamortized discount	0.7	0
Two Thousand Fourteen Senior Notes Thirty Year [Member]
Debt Instrument [Line Items]
Fair value of interest rate swap		0
Unamortized discount	$ 1.6