Investments Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investments available for sale
Amortized Cost	$ 1,836,447	$ 1,629,538
Unrealized Gains	17,027	7,130
Unrealized Losses	(12,433)	(21,566)
Fair Value	1,841,041	1,615,102
U.S. Treasury securities
Investments available for sale
Amortized Cost	205,023	195,990
Unrealized Gains	87	55
Unrealized Losses	(2,746)	(4,497)
Fair Value	202,364	191,548
Municipal debt securities
Investments available for sale
Amortized Cost	362,363	334,048
Unrealized Gains	8,551	3,649
Unrealized Losses	(846)	(3,373)
Fair Value	370,068	334,324
Corporate debt securities
Investments available for sale
Amortized Cost	552,306	457,842
Unrealized Gains	5,628	2,343
Unrealized Losses	(1,969)	(3,828)
Fair Value	555,965	456,357
Residential and commercial mortgage securities
Investments available for sale
Amortized Cost	68,421	68,430
Unrealized Gains	1,524	488
Unrealized Losses	(273)	(582)
Fair Value	69,672	68,336
Asset-backed securities
Investments available for sale
Amortized Cost	135,266	127,359
Unrealized Gains	451	260
Unrealized Losses	(212)	(447)
Fair Value	135,505	127,172
Money market funds
Investments available for sale
Amortized Cost	83,508	102,430
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	83,508	102,430
U.S. Agency \| U.S. agency securities
Investments available for sale
Amortized Cost	26,599	18,785
Unrealized Gains	8	0
Unrealized Losses	(250)	(344)
Fair Value	26,357	18,441
U.S. Agency \| U.S. agency mortgage-backed securities
Investments available for sale
Amortized Cost	402,961	324,654
Unrealized Gains	778	335
Unrealized Losses	(6,137)	(8,495)
Fair Value	$ 397,602	$ 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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