Business Combinations Business Acquisitions - Pro-forma Table (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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
Business Acquisition, Pro Forma Revenue			$ 1,688,297
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax			411,497
Business Acquisition, Pro Forma Net Income (Loss)			$ 248,720
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Basic			$ 1.76
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted			$ 1.73
Pro Forma Weighted Average Shares Outstanding, Diluted			140,112
Two Thousand Seventeen Acquisition [Member] [Domain]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Weighted Average Basic Shares Outstanding, Pro Forma	136,290	136,139
Business Acquisition, Pro Forma Revenue	$ 1,891,701	$ 1,784,776
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax	453,397	429,490
Business Acquisition, Pro Forma Net Income (Loss)	$ 401,908	$ 261,133
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Basic	$ 2.88	$ 1.87
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted	$ 2.83	$ 1.85
Pro Forma Weighted Average Shares Outstanding, Diluted	138,793	137,804