Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Balance at beginning of period	$ 1,413.0	$ 1,681.9	$ 1,620.7
Additions	336.0	46.9	715.6
Terminations	316.2	315.8	654.4
Balance at ending of period	1,432.8	1,413.0	1,681.9
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174.0	174.0	174.0
Additions	0	0	250.0
Terminations	0	0	250.0
Balance at ending of period	174.0	174.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	554.0	617.9	661.9
Additions	0	0	0
Terminations	45.2	63.9	44.0
Balance at ending of period	508.8	554.0	617.9
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	685.0	890.0	780.0
Additions	250.0	0	350.0
Terminations	185.0	205.0	240.0
Balance at ending of period	750.0	685.0	890.0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0	4.8
Additions	86.0	46.9	115.6
Terminations	86.0	46.9	120.4
Balance at ending of period	$ 0	$ 0	$ 0