Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2018	$ 1.3
Amount of Commitment Expiration by Period - 2019	0.2
Amount of Commitment Expiration by Period - 2020	0.2
Amount of Commitment Expiration by Period - 2021	0.2
Amount of Commitment Expiration by Period - 2022	0.2
Amount of Commitment Expiration by Period - Thereafter	17.6
Total Amounts Committed	19.7
Letters of Credit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2018	0.7
Amount of Commitment Expiration by Period - Thereafter	16.5
Total Amounts Committed	17.2
Financial Guarantees [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2018	0.2
Amount of Commitment Expiration by Period - 2019	0.2
Amount of Commitment Expiration by Period - 2020	0.2
Amount of Commitment Expiration by Period - 2021	0.2
Amount of Commitment Expiration by Period - 2022	0.2
Amount of Commitment Expiration by Period - Thereafter	1.1
Total Amounts Committed	2.1
Funding Commitments [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Amount of Commitment Expiration by Period - 2018	0.4
Total Amounts Committed	$ 0.4