Employee Benefit Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 1997	May 05, 2004
Defined Benefit Plan Disclosure [Line Items]
Recognized stock compensation expense	$ 17.4	$ 16.1	$ 11.4
Separation benefit plans expense	2.7	2.4	2.2
Employment contract period extension, years	1 year
Grace period following the first anniversary	30 days
Employee Thrift Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution of shares, common stock	120,333	111,995	95,598
Recognized stock compensation expense	5.2	6.0	5.5
Salary Deferral Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Liability recorded under deferral plan	$ 4.1	$ 3.6
Separation Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service period, years				20 years
Maximum period benefit, weeks				104
Special Separation Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service period, years					20 years
Age limit					65
Change Of Control Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employee contract period, years	3
Multiple for determination compensation	2.9
Plan 401k [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional period for 401k, years	3 years