Summary of Long-Term Debt Outstanding (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Capital Lease Obligations	$ 29.7	$ 31.8
Total Long-Term Debt	1,371.3	1,615.1
Long-Term Debt Qualifying as Risk-Based Capital	$ 909.2	$ 1,009.1
Notes 5.85 Percent Bank Subordinated Debt Due November 2017
Debt Instrument [Line Items]
Interest Rate	5.85%	5.85%
Subordinated Debt	$ 214.5	$ 221.6
Notes 6.50 Percent Bank Subordinated Debt Due August 2018
Debt Instrument [Line Items]
Interest Rate	6.50%	6.50%
Subordinated Debt	$ 326.7	$ 335.0
Notes issued at a discount	0.02%	0.02%
Sterling Denominated Notes 5.375 Percent Bank Subordinated Debt Due March 2015
Debt Instrument [Line Items]
Interest Rate	5.375%	5.375%
Subordinated Debt	$ 0.0	$ 233.7
Notes issued at a discount	0.484%	0.484%
Bank Subordinated Debt [Member]
Debt Instrument [Line Items]
Subordinated Debt	$ 541.2	$ 790.3
Corporation-Subordinated 3.95% Notes due Oct. 2025
Debt Instrument [Line Items]
Interest Rate	3.95%	3.95%
Subordinated Debt	$ 800.4	$ 793.0
Notes issued at a discount	0.114%	0.114%