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Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Net Income, Gross |  |  |  |  |  |  |  |  | $ 232,692,000 | $ 197,131,000 | $ 142,267,000
Net Income, Tax |  |  |  |  |  |  |  |  | 66,831,000 | 55,304,000 | 35,849,000
Net income | $ 45,389,000 | $ 46,996,000 | $ 33,768,000 | $ 39,708,000 | $ 41,350,000 | $ 53,162,000 | $ 29,341,000 | $ 17,974,000 | 165,861,000 | 141,827,000 | 106,418,000
Unrealized holding (losses) gains during the period, Gross |  |  |  |  |  |  |  |  | (40,221,000) | 72,940,000 | (83,934,000)
Unrealized holding (losses) gains during the period, Tax |  |  |  |  |  |  |  |  | (14,078,000) | 25,529,000 | (29,377,000)
Unrealized holding (losses) gains arising during period |  |  |  |  |  |  |  |  | (26,143,000) | 47,411,000 | (54,557,000)
Non-credit portion of other-than-temporary impairment losses recognized in other comprehensive income, Gross |  |  |  |  |  |  |  |  |  |  | 77,000
Non-credit portion of other-than-temporary impairment losses recognized in other comprehensive income, Tax |  |  |  |  |  |  |  |  |  |  | 27,000
Non-credit portion of other-than-temporary impairments recognized in other comprehensive income |  |  |  |  |  |  |  |  |  |  | 50,000
Amount reclassified into net income: HTM securities, Gross |  |  |  |  |  |  |  |  | (580,000) | (1,299,000) | (1,577,000)
Amortization Of Net Unrealized Gains Losses On Held-to-Maturity Transferred From Available-for-Sale Tax |  |  |  |  |  |  |  |  | (203,000) | (455,000) | (552,000)
Amount reclassified from AOCI, HTM |  |  |  |  |  |  |  |  | (377,000) | (844,000) | (1,025,000)
Non-credit OTTI, Gross |  |  |  |  |  |  |  |  | 357,000 | 1,669,000 | 14,000
Non-credit OTTI, Tax |  |  |  |  |  |  |  |  | 125,000 | 584,000 | 5,000
Amounts reclassified into net income: Non-credit other-than-temporary impairment |  |  |  |  |  |  |  |  | 232,000 | 1,085,000 | 9,000
Realized gains on AFS securities, Gross |  |  |  |  |  |  |  |  | (14,016,000) | (28,864,000) | (23,540,000)
Realized gains on AFS securities, Tax |  |  |  |  |  |  |  |  | (4,906,000) | (10,102,000) | (8,239,000)
Realized gains on available for sale securities |  |  |  |  |  |  |  |  | (9,110,000) | (18,762,000) | (15,301,000)
Net unrealized (losses) gains, Gross |  |  |  |  |  |  |  |  | (54,460,000) | 44,446,000 | (108,960,000)
Net unrealized (losses) gains, Tax |  |  |  |  |  |  |  |  | (19,062,000) | 15,556,000 | (38,136,000)
Total unrealized (losses) gains on investment securities |  |  |  |  |  |  |  |  | (35,398,000) | 28,890,000 | (70,824,000)
Net actuarial gain (loss), Gross |  |  |  |  |  |  |  |  | 2,438,000 | (54,136,000) | 59,654,000
Net actuarial gain (loss), Tax |  |  |  |  |  |  |  |  | 853,000 | (18,947,000) | 20,879,000
Net actuarial gain (loss) |  |  |  |  |  |  |  |  | 1,585,000 | (35,189,000) | 38,775,000
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax |  |  |  |  |  |  |  |  | 7,077,000 | 1,902,000 | 4,374,000
Amount reclassified into Net Income: Net actuarial loss, Tax |  |  |  |  |  |  |  |  | 2,477,000 | 666,000 | 1,531,000
Amount reclassified into net income: Net actuarial loss |  |  |  |  |  |  |  |  | 4,600,000 | 1,236,000 | 2,843,000
Amount reclassified into Net Income: Prior service cost, Gross |  |  |  |  |  |  |  |  |  |  | 10,000
Amount reclassified into Net Income: Prior service cost, Tax |  |  |  |  |  |  |  |  |  |  | 4,000
Amount reclassified into net income: Prior service cost |  |  |  |  |  |  |  |  | 0 | 0 | 6,000
Other Comprehensive Income Loss Amortization Pension And Other Postretirement Benefit Plans Curtailment Expense Recognized In Net Periodic Benefit Cost Before Tax |  |  |  |  |  |  |  |  |  |  | 16,000
Other Comprehensive Income Loss Amortization Pension And Other Postretirement Benefit Plans Curtailment Expense Recognized In Net Periodic Benefit Cost Tax |  |  |  |  |  |  |  |  |  |  | 5,000
Other Comprehensive Income Loss Amortization Pension And Other Post Retirement Benefit Plans Curtailment Expense Recognized In Net Periodic Benefit Cost Net of Tax |  |  |  |  |  |  |  |  | 0 | 0 | 11,000
Defined pension and other post-retirement benefit plans, Gross |  |  |  |  |  |  |  |  | 9,515,000 | (52,234,000) | 64,054,000
Defined pension and other post-retirement benefit plans, Tax |  |  |  |  |  |  |  |  | 3,330,000 | (18,281,000) | 22,419,000
Total defined benefit pension and post-retirement plans |  |  |  |  |  |  |  |  | 6,185,000 | (33,953,000) | 41,635,000
Other Comprehensive Loss, Gross |  |  |  |  |  |  |  |  | (44,945,000) | (7,788,000) | (44,906,000)
Comprehensive Income, Tax |  |  |  |  |  |  |  |  | 51,099,000 | 52,579,000 | 20,132,000
Other Comprehensive Loss, Tax |  |  |  |  |  |  |  |  | (15,732,000) | (2,725,000) | (15,717,000)
Other comprehensive loss | (3,386,000) | 6,290,000 | (35,944,000) | 3,827,000 | (29,337,000) | (18,887,000) | 26,483,000 | 16,678,000 | (29,213,000) | (5,063,000) | (29,189,000)
Comprehensive Income, Before Tax |  |  |  |  |  |  |  |  | 187,747,000 | 189,343,000 | 97,361,000
Comprehensive income | $ 42,003,000 | $ 53,286,000 | $ (2,176,000) | $ 43,535,000 | $ 12,013,000 | $ 34,275,000 | $ 55,824,000 | $ 34,652,000 | 136,648,000 | 136,764,000 | 77,229,000
Accumulated Other-than-Temporary Impairment [Member] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive loss |  |  |  |  |  |  |  |  | 232,000 | 1,085,000 |
Accumulated Defined Benefit Plans Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |
Total defined benefit pension and post-retirement plans |  |  |  |  |  |  |  |  | 4,600,000 | 1,236,000 |
Accumulated other comprehensive income (loss) [Member] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive loss |  |  |  |  |  |  |  |  | $ (29,213,000) | $ (5,063,000) | $ (29,189,000)

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