Guarantees and Off-Balance Sheet Arrangements (Details)	Sep. 30, 2018 USD ($)
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	$ 370,000,000
Line of credit facility, remaining borrowing capacity	370,000,000
Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	300,000,000
Line of credit facility, remaining borrowing capacity	300,000,000
Texas Capital Bank | Pledge Lines
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	75,000,000
Line of credit facility, remaining borrowing capacity	75,000,000
BMO Harris Bank | 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 | Tri-Party Pledge Line
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	75,000,000
Line of credit facility, remaining borrowing capacity	75,000,000
BMO Harris Bank | Revolving Credit Facility
Debt Instrument [Line Items]
Letter of credit, borrowing capacity	70,000,000
Line of credit facility, remaining borrowing capacity	$ 70,000,000