Employee Benefit Plans (Details) $ in Millions			12 Months Ended
	May. 05, 2004 yr	Jan. 01, 1997 week	Dec. 31, 2015 USD ($) yr shares		Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares
Defined Benefit Plan Disclosure [Line Items]
Recognized stock compensation expense			$ 15.3	[1]	$ 17.4	$ 16.1
Employment contract period extension, years			1 year
Grace period following the first anniversary			30 days
Employee Thrift Plan
Defined Benefit Plan Disclosure [Line Items]
Contribution of shares, common stock | shares			235,104		120,333	111,995
Recognized stock compensation expense			$ 6.2		$ 5.2	$ 6.0
Salary Deferral Plan
Defined Benefit Plan Disclosure [Line Items]
Liability recorded under deferral plan			4.2		4.1
Separation Plan
Defined Benefit Plan Disclosure [Line Items]
Service period, years		20 years
Maximum period benefit, weeks | week		104
Separation benefit plans expense			$ 3.0		$ 2.7	$ 2.4
Special Separation Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Service period, years	20 years
Age limit | yr	65
Change Of Control Contracts
Defined Benefit Plan Disclosure [Line Items]
Employee contract period, years | yr			3
Multiple for determination compensation			2.9
Plan 401k
Defined Benefit Plan Disclosure [Line Items]
Additional period for 401k, years			3 years

[1] Recognized stock compensation was reduced by $3.2 million, capitalized stock compensation cost for our oil and natural gas properties was reduced by $0.2 million, and the tax benefit was reduced by $1.2 million for lower expected payouts related to the performance shares.