Investments Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investments available for sale
Amortized Cost	$ 1,348,767	$ 1,275,972
Unrealized Gains	23,488	9,535
Unrealized Losses	(3,753)	(8,873)
Fair Value	1,368,502	1,276,634
U.S. Treasury securities
Investments available for sale
Amortized Cost	208,462	178,460
Unrealized Gains	3,329	235
Unrealized Losses	(11)	(1,088)
Fair Value	211,780	177,607
U.S. agency securities
Investments available for sale
Amortized Cost	14,548	13,955
Unrealized Gains	109	5
Unrealized Losses		(178)
Fair Value	14,657	13,782
Municipal debt securities
Investments available for sale
Amortized Cost	293,448	272,733
Unrealized Gains	10,853	7,357
Unrealized Losses	(190)	(262)
Fair Value	304,111	279,828
Corporate debt securities
Investments available for sale
Amortized Cost	411,527	399,246
Unrealized Gains	6,276	1,338
Unrealized Losses	(1,115)	(3,852)
Fair Value	416,688	396,732
Mortgage-backed securities
Investments available for sale
Amortized Cost	53,171	56,380
Unrealized Gains	633	97
Unrealized Losses	(758)	(1,121)
Fair Value	53,046	55,356
Asset-backed securities
Investments available for sale
Amortized Cost	123,123	127,919
Unrealized Gains	203	29
Unrealized Losses	(1,597)	(1,319)
Fair Value	121,729	126,629
Money market funds
Investments available for sale
Amortized Cost	68,488	67,098
Fair Value	68,488	67,098
U.S. Agency \| Mortgage backed securities
Investments available for sale
Amortized Cost	176,000	160,181
Unrealized Gains	2,085	474
Unrealized Losses	(82)	(1,053)
Fair Value	$ 178,003	$ 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
-Paragraph 1
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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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