Investments Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments available for sale
Amortized Cost	$ 1,399,689	$ 1,275,972
Unrealized Gains	37,408	9,535
Unrealized Losses	(1,923)	(8,873)
Fair Value	1,435,174	1,276,634
U.S. Treasury securities
Investments available for sale
Amortized Cost	195,757	178,460
Unrealized Gains	6,046	235
Unrealized Losses	(1)	(1,088)
Fair Value	201,802	177,607
U.S. agency securities
Investments available for sale
Amortized Cost	14,140	13,955
Unrealized Gains	199	5
Unrealized Losses	0	(178)
Fair Value	14,339	13,782
Municipal debt securities
Investments available for sale
Amortized Cost	313,847	272,733
Unrealized Gains	16,405	7,357
Unrealized Losses	(49)	(262)
Fair Value	330,203	279,828
Corporate debt securities
Investments available for sale
Amortized Cost	380,882	399,246
Unrealized Gains	9,828	1,338
Unrealized Losses	(285)	(3,852)
Fair Value	390,425	396,732
Mortgage-backed securities
Investments available for sale
Amortized Cost	50,005	56,380
Unrealized Gains	1,078	97
Unrealized Losses	(479)	(1,121)
Fair Value	50,604	55,356
Asset-backed securities
Investments available for sale
Amortized Cost	129,995	127,919
Unrealized Gains	400	29
Unrealized Losses	(1,083)	(1,319)
Fair Value	129,312	126,629
Money market funds
Investments available for sale
Amortized Cost	114,235	67,098
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	114,235	67,098
U.S. Agency \| Mortgage backed securities
Investments available for sale
Amortized Cost	200,828	160,181
Unrealized Gains	3,452	474
Unrealized Losses	(26)	(1,053)
Fair Value	$ 204,254	$ 159,602

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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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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