Investments Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investments available for sale
Amortized Cost	$ 1,275,972	$ 1,050,901
Unrealized Gains	9,535	9,500
Unrealized Losses	(8,873)	(2,788)
Fair Value	1,276,634	1,057,613
U.S. Treasury securities
Investments available for sale
Amortized Cost	178,460	73,432
Unrealized Gains	235	927
Unrealized Losses	(1,088)	(143)
Fair Value	177,607	74,216
U.S. agency securities
Investments available for sale
Amortized Cost	13,955	4,491
Unrealized Gains	5	29
Unrealized Losses	(178)
Fair Value	13,782	4,520
Municipal debt securities
Investments available for sale
Amortized Cost	272,733	191,723
Unrealized Gains	7,357	4,147
Unrealized Losses	(262)	(324)
Fair Value	279,828	195,546
Corporate debt securities
Investments available for sale
Amortized Cost	399,246	295,507
Unrealized Gains	1,338	2,123
Unrealized Losses	(3,852)	(801)
Fair Value	396,732	296,829
Mortgage-backed securities
Investments available for sale
Amortized Cost	56,380	66,396
Unrealized Gains	97	574
Unrealized Losses	(1,121)	(884)
Fair Value	55,356	66,086
Asset-backed securities
Investments available for sale
Amortized Cost	127,919	126,474
Unrealized Gains	29	136
Unrealized Losses	(1,319)	(422)
Fair Value	126,629	126,188
Money market funds
Investments available for sale
Amortized Cost	67,098	210,688
Fair Value	67,098	210,688
U.S. Agency \| U.S. agency mortgage-backed securities
Investments available for sale
Amortized Cost	160,181	82,190
Unrealized Gains	474	1,564
Unrealized Losses	(1,053)	(214)
Fair Value	$ 159,602	$ 83,540

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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