Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
Minimum | Buildings
Property and equipment [Line items]
Useful life	30 years
Minimum | Technology equipment
Property and equipment [Line items]
Useful life	3 years
Minimum | Furniture, fixtures and other equipment
Property and equipment [Line items]
Useful life	3 years
Minimum | Software
Property and equipment [Line items]
Useful life	3 years
Minimum | Purchased software
Property and equipment [Line items]
Useful life	3 years
Minimum | Capitalized software development costs for internal use
Property and equipment [Line items]
Useful life	3 years
Minimum | Capitalized software development costs for sale or license to third parties
Property and equipment [Line items]
Useful life	3 years
Maximum | Buildings
Property and equipment [Line items]
Useful life	40 years
Maximum | Technology equipment
Property and equipment [Line items]
Useful life	5 years
Maximum | Furniture, fixtures and other equipment
Property and equipment [Line items]
Useful life	5 years
Maximum | Software
Property and equipment [Line items]
Useful life	5 years
Maximum | Purchased software
Property and equipment [Line items]
Useful life	5 years
Maximum | Capitalized software development costs for internal use
Property and equipment [Line items]
Useful life	5 years
Maximum | Capitalized software development costs for sale or license to third parties
Property and equipment [Line items]
Useful life	5 years