Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Jun. 30, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 72.1
30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	53.8
60-89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.3
Monthly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Monthly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Monthly | 60-89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Quarterly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Quarterly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Quarterly | 60-89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Semi-annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	72.1
Semi-annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	53.8
Semi-annual | 60-89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.3
Annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Annual | 60-89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 0.0
September 2018
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Investments	1
Other long-term investments	$ 53.8