Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
Share-based compensation tax benefit (deficiency)	$ 1,656	$ (3,168)	$ 317
Valuation allowance recorded during the period	589	2,273	$ 4,083
Deferred tax assets expected to be realized	511,900
Valuation allowance	11,713	12,303
Net deferred income taxes	511,948	622,464
Deferred tax assets, not subject to expiration	155,600
Tax credit carryforwards	59,434	52,194
Other tax credits	28,950	34,576
Unrecognized tax benefits, income tax penalties and interest accrued	96	105
Unrecognized tax benefits that would impact effective tax rate	8,300	8,500
Unrecognized tax benefits, income tax penalties and interest accrued that would impact effective tax rate	63	$ 68
Approximate range of uncertain income tax positions expected to be settled or resolved during the next 12 months, minimum	320
Period through 2019 [Member]
Income Tax Contingency [Line Items]
Valuation allowance	11,700
Period from 2030 through 2032 [Member]
Income Tax Contingency [Line Items]
Deferred tax assets, subject to expiration	256,800
Period through 2035 [Member]
Income Tax Contingency [Line Items]
Deferred tax assets, subject to expiration	51,800
Federal [Member]
Income Tax Contingency [Line Items]
Alternative minimum tax credits	31,700
Federal and State [Member] | Period through 2035 [Member]
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	$ 27,700