Business Acquisitions - Narrative (Details) - USD ($)	3 Months Ended	6 Months Ended
	Mar. 31, 2016	Jun. 30, 2016	Jun. 30, 2015
Business Acquisition [Line Items]
Acquisition of businesses, net of cash acquired		$ 171,600,000	$ 127,000,000
Goodwill recognized through acquisitions		$ 64,000,000
R.W. Lyall & Company, Inc.
Business Acquisition [Line Items]
Acquisition of businesses, net of cash acquired	$ 129,100,000
Intangible assets	69,600,000
Goodwill recognized through acquisitions	$ 47,500,000
Weighted average useful life	21 years
EMC
Business Acquisition [Line Items]
Acquisition of businesses, net of cash acquired	$ 42,500,000
Intangible assets	16,900,000
Goodwill recognized through acquisitions	$ 16,500,000
Weighted average useful life	19 years
Goodwill, expected to be tax deductible	$ 0