Basis of Presentation and Summary of Significant Accounting Policies - Reclassification of Private Equity Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Equity method investments
Other assets	$ 124,021	$ 84,016
Equity investments	$ 163,518	133,556
Atlanta-based venture capital fund limited partnership private equity
Equity method investments
Equity investments		24,500
Atlanta-based venture capital fund limited partnership private equity \| Reclassification adjustment
Equity method investments
Other assets		$ (24,500)

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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