Investments Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investments available for sale
Amortized Cost	$ 1,732,058	$ 1,629,538
Unrealized Gains	10,394	7,130
Unrealized Losses	(17,919)	(21,566)
Fair Value	1,724,533	1,615,102
U.S. Treasury securities
Investments available for sale
Amortized Cost	182,445	195,990
Unrealized Gains	141	55
Unrealized Losses	(3,843)	(4,497)
Fair Value	178,743	191,548
U.S. agency securities
Investments available for sale
Amortized Cost	26,607	18,785
Unrealized Gains	0	0
Unrealized Losses	(323)	(344)
Fair Value	26,284	18,441
Municipal debt securities
Investments available for sale
Amortized Cost	358,581	334,048
Unrealized Gains	4,967	3,649
Unrealized Losses	(2,156)	(3,373)
Fair Value	361,392	334,324
Corporate debt securities
Investments available for sale
Amortized Cost	504,700	457,842
Unrealized Gains	3,558	2,343
Unrealized Losses	(2,970)	(3,828)
Fair Value	505,288	456,357
Residential and commercial mortgage securities
Investments available for sale
Amortized Cost	64,768	68,430
Unrealized Gains	941	488
Unrealized Losses	(421)	(582)
Fair Value	65,288	68,336
Asset-backed securities
Investments available for sale
Amortized Cost	134,984	127,359
Unrealized Gains	345	260
Unrealized Losses	(418)	(447)
Fair Value	134,911	127,172
Money market funds
Investments available for sale
Amortized Cost	97,388	102,430
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	97,388	102,430
U.S. Agency \| U.S. agency mortgage-backed securities
Investments available for sale
Amortized Cost	362,585	324,654
Unrealized Gains	442	335
Unrealized Losses	(7,788)	(8,495)
Fair Value	$ 355,239	$ 316,494

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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