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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Tax benefit of change in postretirement benefit plans $ 0 $ 399.0 $ (18.0)
Tax expense of net change in unrealized gains on investment securities 1.0 7.8 (0.3)
Tax (expense) benefit of prior service gain from plan amendment 0 (1.0) 1.1
Tax benefit (expense) of net actuarial gain (loss) arising during the period 0 443.1 14.5
Tax benefit of amortization of prior service costs 0 (1.0) (0.9)
Tax benefit of amortization of net actuarial loss 0 (42.1) (32.7)
Tax benefit (expense) of Unrealized holding (losses) gains arising during period 0 1.7 (2.8)
Tax expense of gains realized during the period $ 1.0 $ 6.1 $ 3.1