Acquisitions and Equity Investments - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Nov. 02, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 2,153,988		$ 956,153	$ 840,582
Alliance Automotive Group [Member]
Business Acquisition [Line Items]
Trade accounts receivable		$ 380,000
Merchandise inventories		374,000
Prepaid expenses and other current assets		213,000
Intangible assets		727,000
Deferred tax assets		4,000
Other assets		25,000
Property and equipment		93,000
Total identifiable assets acquired		1,816,000
Current liabilities		(768,000)
Long-term debt		(769,000)
Pension and other post-retirement benefit liabilities		(14,000)
Deferred tax liabilities		(151,000)
Other long-term liabilities		(32,000)
Total liabilities assumed		(1,734,000)
Net identifiable assets acquired		82,000
Noncontrolling interests in subsidiaries		(38,000)
Goodwill		1,036,000
Net assets acquired		$ 1,080,000