Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions	Dec. 31, 2017 USD ($) fund	Dec. 31, 2016 USD ($)
Long/short equity REIT
Investment securities
Carrying value	$ 20.8
Hedge funds
Investment securities
Number of investments held | fund	1
Private equity funds
Investment securities
Number of investments held | fund	10
Carrying value	$ 70.4
Total hedge and private equity funds included in other long-term investments
Investment securities
Other long-term investments largest single investment	54.9	$ 21.5
Carrying value	125.3	82.6
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 99.3	123.6
Hedge funds | Long/short banks and financial
Investment securities
Number of investments held | fund	1
Carrying value	$ 54.9	21.5
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Hedge funds | Long/short equity REIT
Investment securities
Carrying value	0.0	19.9
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Investment securities
Carrying value	54.9	41.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Private equity funds | Private equity funds
Investment securities
Carrying value	70.4	41.2
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	99.3	123.6
Private equity funds | Manufacturing/Industrial
Investment securities
Carrying value	43.3	19.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	10.4	22.9
Private equity funds | Aerospace/Defense/Government
Investment securities
Carrying value	15.8	19.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	12.9	25.9
Private equity funds | Direct lending
Investment securities
Carrying value	7.1	1.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	23.1	28.6
Private equity funds | Financial Services
Investment securities
Carrying value	4.2	1.0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	11.7	5.0
Private equity funds | Insurance
Investment securities
Carrying value	0.0	0.0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 41.2	$ 41.2
Lock-up period expiring 2018 | Hedge funds
Investment securities
Number of investments held | fund	1
Carrying value	$ 54.9