Debt (Narrative) (Detail) (USD $)	6 Months Ended	1 Months Ended		1 Months Ended		9 Months Ended		9 Months Ended	1 Months Ended
	Jun. 30, 2012	Apr. 30, 2012 Amended Credit Agreement [Member]	Sep. 30, 2012 Amended Credit Agreement [Member]	Aug. 31, 2012 5.8% Senior Notes [Member]	Jun. 30, 2012 4% Senior Notes Due June 15, 2022 [Member]	Sep. 30, 2012 CNA [Member] Credit Agreements [Member]	Apr. 30, 2012 CNA [Member] Credit Agreements [Member]	Sep. 30, 2012 Diamond Offshore [Member]	Sep. 30, 2012 Boardwalk Pipeline [Member]	Dec. 31, 2011 Boardwalk Pipeline [Member]
Debt Instrument [Line Items]
Revolving credit facility maximum borrowing capacity		$ 1,000,000,000					$ 250,000,000	$ 750,000,000
Maturity date	Jun. 15, 2015	Apr. 30, 2017			Jun. 15, 2022	Apr. 19, 2016			Dec. 01, 2016
Additional available increase in borrowing capacity							100,000,000
Credit facility maturity date								Sep. 28, 2017
Variable rate term loan									200,000,000
Repayment of debt				225,000,000
Interest rate	4.30%		1.30%	5.80%	4.00%
Principal amount					300,000,000
Revolving credit facility, outstanding amount			350,000,000
Available borrowing capacity			650,000,000
Total debt balance amount									3,200,000,000	3,400,000,000
Acquisition, borrowings funded	81,000,000
Loan funded for acquisition	$ 105,000,000