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Investment Securities (Narrative) (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2016
USD ($)
securities
Dec. 31, 2015
USD ($)
Mar. 31, 2015
USD ($)
Available-for-sale Securities and Held-to-maturity Securities [Abstract]      
Available-for-sale securities, transferred to held-to-maturity securities $ 0.0 $ 862.7  
Available-for-sale securities, transferred to held-to-maturity, unrealized gain (loss)   14.4  
Pledged Securities, carrying value $ 1,200.0 $ 1,200.0 $ 1,100.0
Number of securities by a single non-goverment sponsored issuer exceeding 10% of shareholders' equity | securities 0