Summary of Significant Accounting Policies - Goodwill and Other Intangible Assets and Liabilities - Finite-Lived Intangible Assets and Liabilities Narrative (Details)	9 Months Ended
	Sep. 30, 2018
Customer Relationships | Minimum
Finite-Lived Intangible Assets
Amortization period	10 years
Customer Relationships | Maximum
Finite-Lived Intangible Assets
Amortization period	30 years
Customer inducements | Minimum
Finite-Lived Intangible Assets
Amortization period	10 years
Customer inducements | Maximum
Finite-Lived Intangible Assets
Amortization period	30 years
Customer Inducements, Current Record Management Vendor Or Payments To Customers [Member] | Minimum
Finite-Lived Intangible Assets
Amortization period	5 years
Customer Inducements, Current Record Management Vendor Or Payments To Customers [Member] | Maximum
Finite-Lived Intangible Assets
Amortization period	15 years