Stockholders' Equity - Dividends Declared and Paid (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
Equity [Abstract]
Cash dividend declared per Class A and Class B share (in dollars per share)	$ 0.29	$ 0.29	$ 0.29	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.2
Total amount of dividends paid (in millions)	$ 6.8	$ 7.0	$ 7.2	$ 6.7	$ 6.7	$ 6.7	$ 6.8	$ 6.3	$ 6.3	$ 6.3	$ 6.4	$ 5.1

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration