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Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
U.S. Treasury obligations [Member]
Investments held in an unrealized loss position
Fair Value	$ 82,630	$ 79,833
Unrealized Loss	818	911
Less than 12 months, Fair Value	82,630	79,833
Less than 12 months, Unrealized Loss	818	911
More than 12 months, Fair Value	0	0
More than 12 months, Unrealized Loss	0	0
US Government-sponsored Enterprises Debt Securities [Member]
Investments held in an unrealized loss position
Fair Value	10,507	11,746
Unrealized Loss	161	191
Less than 12 months, Fair Value	10,507	11,746
Less than 12 months, Unrealized Loss	161	191
More than 12 months, Fair Value	0	0
More than 12 months, Unrealized Loss	0	0
State and municipal bonds [Member]
Investments held in an unrealized loss position
Fair Value	167,434	224,884
Unrealized Loss	4,717	6,952
Less than 12 months, Fair Value	160,343	219,276
Less than 12 months, Unrealized Loss	4,132	6,444
More than 12 months, Fair Value	7,091	5,608
More than 12 months, Unrealized Loss	585	508
Corporate debt [Member]
Investments held in an unrealized loss position
Fair Value	416,640	469,632
Unrealized Loss	6,979	8,480
Less than 12 months, Fair Value	382,246	424,721
Less than 12 months, Unrealized Loss	4,361	5,662
More than 12 months, Fair Value	34,394	44,911
More than 12 months, Unrealized Loss	2,618	2,818
Residential mortgage-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	97,495	103,680
Unrealized Loss	1,956	2,046
Less than 12 months, Fair Value	95,381	100,542
Less than 12 months, Unrealized Loss	1,922	1,982
More than 12 months, Fair Value	2,114	3,138
More than 12 months, Unrealized Loss	34	64
Agency commercial mortgage-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	4,571	4,579
Unrealized Loss	102	143
Less than 12 months, Fair Value	4,191	4,192
Less than 12 months, Unrealized Loss	72	114
More than 12 months, Fair Value	380	387
More than 12 months, Unrealized Loss	30	29
Other commercial mortgage-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	10,890	9,822
Unrealized Loss	106	137
Less than 12 months, Fair Value	9,075	9,179
Less than 12 months, Unrealized Loss	85	134
More than 12 months, Fair Value	1,815	643
More than 12 months, Unrealized Loss	21	3
Other asset-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	46,008	44,343
Unrealized Loss	213	267
Less than 12 months, Fair Value	42,018	39,079
Less than 12 months, Unrealized Loss	207	256
More than 12 months, Fair Value	3,990	5,264
More than 12 months, Unrealized Loss	6	11
Debt Securities [Member]
Investments held in an unrealized loss position
Fair Value	836,175	948,519
Unrealized Loss	15,052	19,127
Less than 12 months, Fair Value	786,391	888,568
Less than 12 months, Unrealized Loss	11,758	15,694
More than 12 months, Fair Value	49,784	59,951
More than 12 months, Unrealized Loss	$ 3,294	$ 3,433

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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