Accounting Policies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounting Policies [Line Items]
Increased cash provided by operating activities and decreased cash used in investing activities		$ 17.5	$ 5.5
Percentage of taxable income to be distributed	100.00%
Minimum [Member]
Accounting Policies [Line Items]
Interest owned on hedged borrowings rest to market rates (in days)	30
Maximum [Member]
Accounting Policies [Line Items]
Interest owned on hedged borrowings rest to market rates (in days)	90