Financing Arrangements Revolving Credit Agreement (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Apr. 04, 2016
Line of Credit Facility [Line Items]
Interest Expense, Debt, Excluding Amortization	$ 2,032,000	$ 2,032,000	$ 6,096,000	$ 4,063,000
Cash Institution Covenant	200,000,000		200,000,000
Proceeds from Lines of Credit			0	0
Line of Credit Facility, Maximum Borrowing Capacity	250,000,000		250,000,000
Debt Instrument, Interest Rate, Stated Percentage						3.25%
Unrestricted Cash Covenant	400,000,000		400,000,000
Letters of Credit Outstanding, Amount	16,500,000		16,500,000		$ 11,100,000
Amortization of Debt Discount (Premium)	2,722,000	2,458,000	7,964,000	4,854,000
Interest Expense, Debt	4,754,000	$ 4,490,000	14,060,000	$ 8,917,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 45,000,000		$ 45,000,000
Minimum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.25%
Maximum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.40%
Geographic Distribution, Domestic [Member]
Line of Credit Facility [Line Items]
Securities Owned and Pledged as Collateral, Description	1
Geographic Distribution, Foreign [Member]
Line of Credit Facility [Line Items]
Securities Owned and Pledged as Collateral, Description	0.65
Base Rate [Member] | Minimum [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.50%		0.50%
Base Rate [Member] | Maximum [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.25%		2.25%