Basis of Presentation - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Basis of Presentation [Line Items]
Additional excess tax benefits	$ 2,864
Net cash provided by financing activities	59,748	$ (102,969)
Net cash provided by operating activities	$ 237,204	164,836
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	$ 2,745	3,321
Net cash provided by operating activities	2,604	$ 3,233
Pro Forma [Member] \| Adjustments for new accounting pronouncements
Basis of Presentation [Line Items]
After-tax unrealized gains on equity securities reclassified from AOCI	$ 15,718

[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.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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