Investments - Schedule of Investments Recorded in Consolidated Balance Sheets (Parenthetical) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Short-term investments
Schedule Of Held To Maturity Securities [Line Items]
Investments serve as collateral for certain obligations	$ 9.1	$ 299.3

The carrying amount of held-to-maturity securities that serve as collateral for borrowings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5086-111524

Name:	us-gaap_HeldToMaturitySecuritiesPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant