Debt Debt (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount	$ 358,600,000
Line of Credit Facility, Amount Outstanding	14,400,000	0
letters of credit outstanding under credit facility	19,600,000
Maximum borrowing availability subject to limit	166,000,000
Number of Secured Letters of Credit Outstanding under Credit Facility	3
Aggregate Capacity of Secured Letters of Credit under Credit Facility	20,000,000
Uncommitted Letters of Credit	2,300,000
Letters of Credit Outstanding Under Letter of Credit Facilities	8,600,000	13,400,000
Restricted Cash for Secured Letter of Credit Agreements	8,800,000	13,700,000
Low Range Uncommitted Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum available amounts under Letter of Credit Facilities	5,000,000
High Range Uncommittted Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum available amounts under Letter of Credit Facilities	10,000,000
New Unsecured Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	200,000,000
Sub-limit for letters of credit	100,000,000
Optional increase in borrowing availability	25,000,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 225,000,000
Maximum [Member] | New Unsecured Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit Facility, Basis Spread on Variable Rate	3.25%
Minimum [Member] | New Unsecured Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit Facility, Basis Spread on Variable Rate	2.25%