Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative
Balance at beginning of period	$ 1,477.4	$ 1,432.8	$ 1,413.0
Additions	318.1	291.0	336.0
Terminations	108.1	246.4	316.2
Balance at ending of period	1,687.4	1,477.4	1,432.8
Credit Default Swaps
Derivative
Balance at ending of period	97.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	97.0	0	0
Additions	0	97.0	0
Terminations	0	0	0
Balance at ending of period	97.0	97.0	0
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	174.0	174.0
Additions	0	0	0
Terminations	0	24.0	0
Balance at ending of period	150.0	150.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	630.4	508.8	554.0
Additions	250.1	160.0	0
Terminations	40.1	38.4	45.2
Balance at ending of period	840.4	630.4	508.8
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	750.0	685.0
Additions	0	0	250.0
Terminations	0	150.0	185.0
Balance at ending of period	600.0	600.0	750.0
Forwards
Derivative
Balance at beginning of period	0	0	0
Additions	68.0	24.0	86.0
Terminations	68.0	24.0	86.0
Balance at ending of period	0	0	0
Options Held
Derivative
Balance at beginning of period	0	0	0
Additions	0	10.0	0
Terminations	0	10.0	0
Balance at ending of period	$ 0	$ 0	$ 0