Debt (Narrative) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jul. 29, 2016	Jan. 19, 2016	Dec. 01, 2015	Dec. 31, 2014	Sep. 08, 2014	Jul. 23, 2014
Debt Instrument [Line Items]
Extinguishment of debt				$ 238,000,000	$ 245,700,000
Loss on debt retirements	$ 0		$ 0	0	$ 8,584,000
Letters of credit outstanding	196,900,000			196,900,000					$ 212,100,000
Notes payable	21,700,000			$ 21,700,000					22,300,000
Debt Instrument, term				6 years
Amount outstanding	107,508,000			$ 107,508,000					140,241,000
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity											$ 500,000,000
Maximum percent of current credit facility that may be issued in addition				50.00%
Maximum additional issuance				$ 300,000,000
Fair value of amount outstanding	$ 0			0
Revolving Credit Facility Accordion Feature
Debt Instrument [Line Items]
Maximum borrowing capacity											$ 1,000,000,000
Repurchase Agreement | Financial Services
Debt Instrument [Line Items]
Maximum borrowing capacity										$ 175,000,000
Line of Credit Facility, Initiation Date	Jun. 01, 2015	Feb. 02, 2015	Sep. 04, 2015
Line of Credit Facility, Expiration Date	Sep. 02, 2016
Amount outstanding	$ 107,500,000			$ 107,500,000					$ 140,200,000
Maximum
Debt Instrument [Line Items]
Stated interest rate	5.00%			5.00%
Senior Unsecured Term Loan Maturing January 3, 2017 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 500,000,000.0			$ 500,000,000.0
Scenario, Forecast [Member] | Repurchase Agreement | Financial Services
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 200,000,000.0	$ 175,000,000.0	$ 200,000,000.0