Investments Available for Sale (Details 3) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Amortized Cost	$ 1,732,058	$ 1,629,538
Fair Value
Total investments	1,724,533	1,615,102
U.S. Treasury securities
Amortized Cost
Due in 1 year	24,778
Due after 1 but within 5 years	37,896
Due after 5 but within 10 years	106,106
Due after 10 years	13,665
Subtotal	182,445
Amortized Cost	182,445	195,990
Fair Value
Due in 1 year	24,767
Due after 1 but within 5 years	37,684
Due after 5 but within 10 years	103,113
Due after 10 years	13,179
Subtotal	178,743
Total investments	178,743	191,548
U.S. agency securities
Amortized Cost
Due in 1 year	0
Due after 1 but within 5 years	26,607
Subtotal	26,607
Amortized Cost	26,607	18,785
Fair Value
Due in 1 year	0
Due after 1 but within 5 years	26,284
Subtotal	26,284
Total investments	26,284	18,441
Municipal debt securities
Amortized Cost
Due in 1 year	6,280
Due after 1 but within 5 years	110,543
Due after 5 but within 10 years	137,654
Due after 10 years	104,104
Subtotal	358,581
Amortized Cost	358,581	334,048
Fair Value
Due in 1 year	6,290
Due after 1 but within 5 years	110,750
Due after 5 but within 10 years	139,776
Due after 10 years	104,576
Subtotal	361,392
Total investments	361,392	334,324
Corporate debt securities
Amortized Cost
Due in 1 year	54,233
Due after 1 but within 5 years	269,360
Due after 5 but within 10 years	176,638
Due after 10 years	4,469
Subtotal	504,700
Amortized Cost	504,700	457,842
Fair Value
Due in 1 year	54,253
Due after 1 but within 5 years	269,941
Due after 5 but within 10 years	176,686
Due after 10 years	4,408
Subtotal	505,288
Total investments	505,288	456,357
Residential and commercial mortgage securities
Amortized Cost
Amortized Cost	64,768	68,430
Fair Value
Total investments	65,288	68,336
Asset-backed securities
Amortized Cost
Amortized Cost	134,984	127,359
Fair Value
Total investments	134,911	127,172
Money market funds
Amortized Cost
Amortized Cost	97,388	102,430
Fair Value
Total investments	97,388	102,430
U.S. Agency \| U.S. agency mortgage-backed securities
Amortized Cost
Amortized Cost	362,585	324,654
Fair Value
Total investments	$ 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
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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