Debt (Revolving Credit Facility) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Oct. 02, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt [Line Items]
Maximum borrowing capacity	$ 1,100,000,000		$ 1,335,000,000	$ 1,335,000,000
Borrowing base	$ 605,927,000		734,464,000	773,228,000
Incremental Facility extension period	365 days
Maximum net outstanding auction guarantees permitted	$ 600,000,000
Covenants for issuing dividends, minimum aggregate borrowing availability	100,000,000
Covenants for issuing dividends, minimum total liquidity amount	200,000,000
Other assets, noncurrent	5,400,000
New Credit Agreement
Debt [Line Items]
Maximum borrowing capacity	1,100,000,000
Line of credit, sub limit	320,000,000
Maximum increase in borrowing capacity	150,000,000
Aggregate fees paid	21,800,000
Incremental Credit Facility
Debt [Line Items]
Borrowing base	50,000,000
Agency Segment Credit Agreement
Debt [Line Items]
Maximum borrowing capacity	300,000,000		300,000,000	300,000,000
Borrowing base	148,299,000		165,443,000	225,642,000
Line of credit, sub limit	50,000,000
Finance Segment Credit Agreement
Debt [Line Items]
Decrease to borrowing capacity		$ 235,000,000
Maximum borrowing capacity	800,000,000		1,035,000,000	1,035,000,000
Borrowing base	457,628,000		$ 569,021,000	$ 547,586,000
Line of credit, sub limit	$ 270,000,000