Fair Value Measurements Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Level 3 Inputs (Details) - Fair Value, Measurements, Recurring - Level 3 - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Pooled trust preferred securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 422	$ 706
Unrealized adjustment to fair value	285	(286)
Discount accretion	0	2
Balance, end of period	707	422
Single-issuer trust preferred securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	2,450	2,630
Unrealized adjustment to fair value	588	(190)
Discount accretion	12	10
Balance, end of period	3,050	2,450
Auction rate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	97,256	98,059
Unrealized adjustment to fair value	1,217	(1,246)
Discount accretion	195	443
Balance, end of period	$ 98,668	$ 97,256

Fair Value Measurements Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Level 3 Inputs (Details) - Fair Value, Measurements, Recurring - Level 3 - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Pooled trust preferred securities

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]

Balance, beginning of period

$ 422

$ 706

Unrealized adjustment to fair value

285

(286)

Discount accretion