CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss, Net of Deferred Taxes [Member]		Unearned Stock Award Common Stock [Member]	Common Stock Held by BMP [Member]	Treasury Stock, at Cost [Member]	Total
Balance at Dec. 31, 2016	$ 536	$ 278,356	$ 503,539	$ (5,939)		$ (1,932)	$ (6,859)	$ (201,833)	$ 565,868
Balance (in shares) at Dec. 31, 2016	37,455,853
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	$ 0	0	36,459	0		0	0	0	36,459
Other comprehensive income (loss), net of tax	0	0	0	1,228		0	0	0	1,228
Exercise of stock options, net	$ 0	680	0	0		0	0	0	680
Exercise of stock options (in shares)	45,174
Release of shares, net of forfeitures	$ 0	1,325	0	0		(2,874)	(170)	1,917	198
Release of shares, net of forfeitures (in shares)	152,215
Stock-based compensation	$ 0	0	0	0		1,270	0	0	1,270
Shares received to satisfy distribution of retirement benefits	$ 0	(3,687)	0	0		0	4,293	(4,511)	(3,905)
Shares received to satisfy distribution of retirement benefits (in shares)	(230,358)
Cash dividends declared and paid	$ 0	0	(15,761)	0		0	0	0	(15,761)
Balance at Sep. 30, 2017	$ 536	276,674	524,237	(4,711)		(3,536)	(2,736)	(204,427)	586,037
Balance (in shares) at Sep. 30, 2017	37,422,884
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of unrealized gains and losses on marketable equity securities | ASU 2016-01 [Member]	$ 0	0	153	(153)	[1]	0	0	0	0
Balance at Dec. 31, 2017	536	276,730	535,130	(3,641)		(2,894)	(2,736)	(204,558)	598,567
Balance (ASU 2016-01 [Member]) at Dec. 31, 2017	$ 536	276,730	535,283	(3,794)		(2,894)	(2,736)	(204,558)	598,567
Balance (in shares) at Dec. 31, 2017	37,419,070
Balance (in shares) (ASU 2016-01 [Member]) at Dec. 31, 2017	37,419,070
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	$ 0	0	38,848	0		0	0	0	38,848
Other comprehensive income (loss), net of tax	0	0	0	(1,972)		0	0	0	(1,972)
Exercise of stock options, net	$ 1	1,118	0	0		0	0	(165)	954
Exercise of stock options (in shares)	57,327
Release of shares, net of forfeitures	$ 0	1,022	0	0		(3,003)	0	1,994	13
Release of shares, net of forfeitures (in shares)	158,851
Stock-based compensation	$ 0	0	0	0		1,198	0	0	1,198
Shares received to satisfy distribution of retirement benefits	$ 0	(1,152)	0	0		0	1,227	(958)	(883)
Shares received to satisfy distribution of retirement benefits (in shares)	(49,895)
Reclassification of tax effects on other comprehensive income (loss)	$ 0	0	(32)	32		0	0	0	0
Cash dividends declared and paid	0	0	(15,742)	0		0	0	0	(15,742)
Repurchase of common stock	$ 0	0	0	0		0	0	(17,406)	(17,406)
Repurchase of common stock (in shares)	(973,200)
Balance at Sep. 30, 2018	$ 537	$ 277,718	$ 558,357	$ (5,734)		$ (4,699)	$ (1,509)	$ (221,093)	$ 603,577
Balance (in shares) at Sep. 30, 2018	36,612,153

[1] Represents the impact of adopting ASU 2016-01 allowing the reclassification of unrealized gains and losses on available-for-sale equity securities from accumulated other comprehensive income to retained earnings.