Guarantees and Off-Balance Sheet Arrangements (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	$ 305,000
Line of Credit Facility, Remaining Borrowing Capacity	305,000
Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	275,000
Line of Credit Facility, Remaining Borrowing Capacity	275,000
Bank of America [Member] | Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	50,000
Line of Credit Facility, Remaining Borrowing Capacity	50,000
Texas Capital Bank [Member] | Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	75,000
Line of Credit Facility, Remaining Borrowing Capacity	75,000
BMO Harris Bank [Member] | Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	150,000
Line of Credit Facility, Remaining Borrowing Capacity	150,000
BMO Harris Bank [Member] | Revolving Credit Facility
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	30,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 30,000