Guarantees and Off-Balance Sheet Arrangements (Details)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	$ 180,000,000
Line of Credit Facility, Remaining Borrowing Capacity	180,000,000
Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	150,000,000
Line of Credit Facility, Remaining Borrowing Capacity	150,000,000
BMO Harris Bank [Member] | Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	150,000,000
Line of Credit Facility, Remaining Borrowing Capacity	150,000,000
BMO Harris Bank [Member] | Revolving Credit Facility
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	30,000,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 30,000,000