Basis of Presentation - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016
Basis of Presentation [Line Items]
Additional excess tax benefits	$ 2,638
Net cash provided by financing activities	67,085	$ (46,586)
Net cash provided by operating activities	$ 154,453	115,770
Percentage of credit losses on available-for-sale deb securities to total investment portfolio	90.00%
Accounting Standards Update 2016-09, Statutory Tax Withholding Component
Basis of Presentation [Line Items]
Net cash provided by financing activities	$ 2,604	3,233
Net cash provided by operating activities	$ 2,604	$ 3,233

[Line Items] for Basis of Presentation [Table]

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Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount

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Disclosure of accounting policy pertaining to pending accounting standards.

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Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3521-108585

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-Name Accounting Standards Codification
-Topic 230
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-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

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-Name Accounting Standards Codification
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