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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 |

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