Business Combinations Business Acquisitions - Pro-forma Table (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Two Thousand Sixteen Acquisition [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Weighted Average Basic Shares Outstanding, Pro Forma	136,139	137,810
Business Acquisition, Pro Forma Revenue	$ 1,789,790	$ 1,716,592
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax	428,194	414,911
Business Acquisition, Pro Forma Net Income (Loss)	$ 260,346	$ 250,783
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Basic	$ 1.86	$ 1.78
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted	$ 1.84	$ 1.75
Pro Forma Weighted Average Shares Outstanding, Diluted	137,804	140,112
Two Thousand Fifteen Acquisitions [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Weighted Average Basic Shares Outstanding, Pro Forma		137,810	140,944
Business Acquisition, Pro Forma Revenue		$ 1,688,297	$ 1,630,992
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax		411,497	356,426
Business Acquisition, Pro Forma Net Income (Loss)		$ 248,720	$ 217,053
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Basic		$ 1.76	$ 1.50
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted		$ 1.73	$ 1.48
Pro Forma Weighted Average Shares Outstanding, Diluted		140,112	142,891
Two Thousand Fourteen Acquisitions [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Weighted Average Basic Shares Outstanding, Pro Forma			140,944	141,033
Business Acquisition, Pro Forma Revenue			$ 1,630,162	$ 1,520,858
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax			358,229	409,522
Business Acquisition, Pro Forma Net Income (Loss)			$ 218,150	$ 248,628
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Basic			$ 1.51	$ 1.72
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted			$ 1.49	$ 1.70
Pro Forma Weighted Average Shares Outstanding, Diluted			142,891	142,624