Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Derivative [Line Items]
Balance at beginning of period	$ 1,413.0	$ 1,681.9
Additions	35.0	19.9
Terminations	83.9	85.9
Balance at ending of period	1,364.1	1,615.9
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174.0	174.0
Additions	0	0
Terminations	0	0
Balance at ending of period	174.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	554.0	617.9
Additions	0	0
Terminations	3.9	11.0
Balance at ending of period	550.1	606.9
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	685.0	890.0
Additions	0	0
Terminations	45.0	55.0
Balance at ending of period	640.0	835.0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0
Additions	35.0	19.9
Terminations	35.0	19.9
Balance at ending of period	$ 0	$ 0