Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Plan's assets at fair value
Total assets at fair value	$ 191,581	$ 158,823
U.S. equities - large cap [Member]
Pension Plan's assets at fair value
Total assets at fair value	56,388	53,021
U.S. fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value	59,832	52,140
International equities [Member]
Pension Plan's assets at fair value
Total assets at fair value	42,939	30,803
U.S. Equities - Small to Mid Cap [Member]
Pension Plan's assets at fair value
Total assets at fair value	17,004	9,494
Real Estate Investment Trust [Member]
Pension Plan's assets at fair value
Total assets at fair value	7,532	6,598
International fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value	5,806	5,802
Cash [Member]
Pension Plan's assets at fair value
Total assets at fair value	1,205	889
Other [Member]
Pension Plan's assets at fair value
Total assets at fair value	875	76
Level 1 [Member]
Pension Plan's assets at fair value
Total assets at fair value	1,205	889
Level 1 [Member] | U.S. equities - large cap [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 1 [Member] | U.S. fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 1 [Member] | International equities [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 1 [Member] | U.S. Equities - Small to Mid Cap [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 1 [Member] | Real Estate Investment Trust [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 1 [Member] | International fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 1 [Member] | Cash [Member]
Pension Plan's assets at fair value
Total assets at fair value	1,205	889
Level 1 [Member] | Other [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 2 [Member]
Pension Plan's assets at fair value
Total assets at fair value	190,376	157,934
Level 2 [Member] | U.S. equities - large cap [Member]
Pension Plan's assets at fair value
Total assets at fair value	56,388	53,021
Level 2 [Member] | U.S. fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value	59,832	52,140
Level 2 [Member] | International equities [Member]
Pension Plan's assets at fair value
Total assets at fair value	42,939	30,803
Level 2 [Member] | U.S. Equities - Small to Mid Cap [Member]
Pension Plan's assets at fair value
Total assets at fair value	17,004	9,494
Level 2 [Member] | Real Estate Investment Trust [Member]
Pension Plan's assets at fair value
Total assets at fair value	7,532	6,598
Level 2 [Member] | International fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value	5,806	5,802
Level 2 [Member] | Cash [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 2 [Member] | Other [Member]
Pension Plan's assets at fair value
Total assets at fair value	875	76
Level 3 [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | U.S. equities - large cap [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | U.S. fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | International equities [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | U.S. Equities - Small to Mid Cap [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | Real Estate Investment Trust [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | International fixed income [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | Cash [Member]
Pension Plan's assets at fair value
Total assets at fair value
Level 3 [Member] | Other [Member]
Pension Plan's assets at fair value
Total assets at fair value