Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Securities [Member]
Investments held in an unrealized loss position
Fair Value	$ 726,905	$ 948,519
Unrealized Loss	9,528	19,127
Less than 12 months, Fair Value	671,635	888,568
Less than 12 months, Unrealized Loss	6,675	15,694
More than 12 months, Fair Value	55,270	59,951
More than 12 months, Unrealized Loss	2,853	3,433
U.S. Treasury obligations [Member]
Investments held in an unrealized loss position
Fair Value	83,416	79,833
Unrealized Loss	727	911
Less than 12 months, Fair Value	78,080	79,833
Less than 12 months, Unrealized Loss	664	911
More than 12 months, Fair Value	5,336	0
More than 12 months, Unrealized Loss	63	0
US Government-sponsored Enterprises Debt Securities [Member]
Investments held in an unrealized loss position
Fair Value	15,907	11,746
Unrealized Loss	127	191
Less than 12 months, Fair Value	15,907	11,746
Less than 12 months, Unrealized Loss	127	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	111,522	224,884
Unrealized Loss	2,069	6,952
Less than 12 months, Fair Value	103,371	219,276
Less than 12 months, Unrealized Loss	1,441	6,444
More than 12 months, Fair Value	8,151	5,608
More than 12 months, Unrealized Loss	628	508
Corporate debt [Member]
Investments held in an unrealized loss position
Fair Value	372,586	469,632
Unrealized Loss	5,235	8,480
Less than 12 months, Fair Value	336,877	424,721
Less than 12 months, Unrealized Loss	3,167	5,662
More than 12 months, Fair Value	35,709	44,911
More than 12 months, Unrealized Loss	2,068	2,818
Residential mortgage-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	89,610	103,680
Unrealized Loss	1,133	2,046
Less than 12 months, Fair Value	87,491	100,542
Less than 12 months, Unrealized Loss	1,086	1,982
More than 12 months, Fair Value	2,119	3,138
More than 12 months, Unrealized Loss	47	64
Agency commercial mortgage-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	4,568	4,579
Unrealized Loss	62	143
Less than 12 months, Fair Value	4,188	4,192
Less than 12 months, Unrealized Loss	34	114
More than 12 months, Fair Value	380	387
More than 12 months, Unrealized Loss	28	29
Other commercial mortgage-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	9,147	9,822
Unrealized Loss	82	137
Less than 12 months, Fair Value	7,457	9,179
Less than 12 months, Unrealized Loss	65	134
More than 12 months, Fair Value	1,690	643
More than 12 months, Unrealized Loss	17	3
Other asset-backed securities [Member]
Investments held in an unrealized loss position
Fair Value	40,149	44,343
Unrealized Loss	93	267
Less than 12 months, Fair Value	38,264	39,079
Less than 12 months, Unrealized Loss	91	256
More than 12 months, Fair Value	1,885	5,264
More than 12 months, Unrealized Loss	$ 2	$ 11